My line of thinking is this: If you want to kill everyone over 18, it should ideally be linked to something that only those older than 18 are able to do.

Computers are getting smaller, and wearables are becoming more fashionable, it's not unfeasible that within a few decades we'll have implants in our brains for interfacing with computers. As with all technology, this will spread to the point where (almost) everyone will want one, and it could be made compulsory by the governments of the world at some point (optional).

Obviously we won't want to put chips in our kids brains until they're fully developed, so when they inevitably malfunction, killing everyone, only the kids will remain. This could encourage a sort of taboo, surrounding technology, which would explain why even those who grow into adults don't instantly start using all the tech that's still laying around - there might even be a concerted effort to get rid of it all. This could open up some new narratives also, with arguments over the new anti-technology practices that are going around, and inquisition style bands of kids, roaming and casting judgement on anyone accused of using tech.

There's no clear dividing line between a person aged 17 and one aged 18. However, before puberty, the human body has significant differences in the release of hormones.

The primary hormone which stimulates puberty is the Gonadotropin-releasing hormone.

GnRH is a neuro-hormone and stimulates certain behaviour patterns, the hormone influences a person's physiology. It is therefore not unreasonable that a certain compound for which GnRH acts as a catalyst can cause dangerous and even deadly effects on only those with a significant amount of GnRH (in short, everyone who is going through puberty would die first, and then all those who already have done that).

Another possibility is a microbe which binds with one of these hormones as a way of infecting its host.

This could kill off everyone over 13 (except for those who present with Isolated gonadotropin-releasing hormone deficiency). Four years later, the 13 year olds will be 17.

Real-World Immunological Changes Occur At The End Of Puberty

The Thymus is a lymphoid immune system organ that grows during childhood up through the end of puberty, then begins to shrink. It's size and strength while young results in a much larger quantity of T-Cells, which make the bodies of people in this age range more capable of fighting certain types of foreign bodies. This is one of the reasons why Chicken Pox is mostly just inconvenient while younger, but can be deadly while older.

You could have an unknown viral disease that everyone has, but remains latent (walking dead style) because the immune system is strong enough to keep it at bay in the vital areas of the body. As they age and the Thymus begins to shrink and weaken, the virus eventually reaches the point where the body is no longer able to keep it in check, and from then on it begins to rapidly spread, killing the host.

Puberty typically ends around 17-18 for both boys and girls, although there is some variance, and the complete range of developmental effects can still go on for longer (early 20's). Nonetheless, you can interpret the effects on the thymus (and at what point it becomes weak enough) to suit your needs for the story.

With the added specification that the children must be able to grow into adults, just have the virus be a one time thing rather than a latent one, and children's bodies are able to fully defeat it instead of just beating it back, so that the virus dies out completely once all the adults are dead.

This was the plot of a Turkish science fiction novel I read as a child. I cannot recall the name of it, though. In that novel, the superpowers develop the same bioweapon and secretly start to vaccinate their children. Adults are not vaccinated for purposes of maintaining secrecy. When the bioweapons are accidentally released, only children from the two superpowers survive. The author may have been Gülten Dayıoğlu but I am not sure.

(UPDATE: Found a source!)

As humans age, our range of hearing changes slightly and it gets harder for us to hear high-frequency sounds. As a result, there are certain tones (between about 16 and 20kHz) that can only be heard by teenagers - these have been used to create special "mosquito alarms" to deter teenage criminals and loiterers, as well as "Teen Buzz" ringtones that can't be heard by teachers.

This change in hearing range - known as presbycusis - is the only way I can immediately think of that you can reliably distinguish between children and adults, in order to kill the latter while sparing the former. Either:

• A tone that only adults can hear, and that kills them, or;
• A tone that only children can hear, and that somehow spares them from whatever kills all the adults

Since you've stated you want the surviving children to be able to grow up, the tone should only play once, audible across the planet, for as long as it takes for all adults to die. After that, it never plays again.

You could create a unique deadly strand of Mononucleosis. Children only show symptoms of the common cold where as adults can suffer all sorts of health problems.

Infectious mononucleosis (IM), also known as mono, kissing disease, or glandular fever, is an infection commonly caused by the Epstein–Barr virus (EBV).[2] Most people are infected by the virus as children, when the disease produces little or no symptoms. In young adults, the disease often results in fever, sore throat, enlarged lymph nodes in the neck, and tiredness. Most people get better in two to four weeks; however, feeling tired may last for months. The liver or spleen may also become swollen.[3] In less than one percent of cases splenic rupture may occur.[6]

Infectious mononucleosis is usually caused by Epstein–Barr virus (EBV), also known as human herpesvirus 4, which is a member of the herpes virus family. A few other viruses may also cause the disease.[3] It is primarily spread through saliva but can rarely be spread through semen or blood. Spread may occur by objects such as drinking glasses or toothbrushes. Those who are infected can spread the disease weeks before symptoms develop.[2]

From Wiki

In an overpopulated world, deep in the future, life expectancy was too damn high.

The solution? Life defining chips inserted into every human being on Earth.

Yup. A chip, powered by the cloud, that defines when you die. That is until Jonathan (a humanist, wiz-kid, computer hacker) corrupted the system entirely. Although things didn't quite go to plan... a built-in fail safe kicked in and automatically wiped every human over the age of 18 years.

Only now can those left over be free. It is up to this younger generation to run the Earth in a peaceful, self-sustaining manner. Will they succeed?

There are plenty of existing literary examples of this. In fact there's a trope for it!

http://tvtropes.org/pmwiki/pmwiki.php/Main/TeenageWasteland

In my experience this is produced by one of two reasons:

Disease

A disease that targets adults. (e.g. Only Fatal to Adults) Biologically speaking, it could be something that triggers with the onset of puberty. If it's a virus, it could easily have infected the entire population.

Population Control

Similarly, it could also be a form of population control as in

According to TvTropes though, there are more variations: "A society where the old nominally still hold power, but groups of youths have become too powerful to be truly controlled. (e.g. A Clockwork Orange)."

Two pandemics, nineteen or twenty years apart.

The first epidemic is a very mild (or even helpful!), easily transmissible disease that remains contagious for an extended period of time. (But it is not contagious forever, and infants who have the disease are not contagious.) After a year or two, everyone on the planet has antibodies to it, and it burns itself out.

The second epidemic is fatal to people who have antibodies to the first disease, but completely asymptomatic to people who do not. It remains contagious indefinitely.

In this scenario, even if there are surviving adults, they will completely segregate themselves from the children. And they will continue to segregate themselves indefinitely, because any contact with young people is likely to kill the older generation.

Rather than a sickness, you could go for nanotechnology. The apocalypse is the release of nanobots which are programmed to infect and kill adults and use some of their component materials to replicate and spread. The best option which occurs to me for discriminating by age which is tunable to the age which suits the story is to look at telomere length decay. It may not be quite accurate enough for biologists, but it should be good enough for a general audience.

That leaves the question of why someone would develop these nanobots. You may already have some ideas. Off the top of my head, there is active research into reversing telemere decay in order to live forever. The billionaires who develop this treatment may wish to dispose of other adults in order to more easily shape society and rule either openly or from the shadows. Whether they survive as planned or not is up to you...

A virus that cuts off Telomeres that are too short.

(Repost from duplicate question that got closed)

To quote Wikipedia: "A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. [...] For vertebrates, the sequence of nucleotides in telomeres is TTAGGG. This sequence of TTAGGG is repeated approximately 2,500 times in humans."

Very roughly speaking: when cells replicate, the telomeres are shortened. This means that in young organisms, telomeres are long, and in older ones they are shorter.

So... assume a virus - like a common flu virus - that has been mutated and targets cells with too short telomeres and simply cuts them off. Cell replication will be shot to hell and your chromosomes in each cell afflicted by the virus will be a jumble; the cells die. Once it hits it will probably be like a severe hemorrhagic fever (like Ebola or Lassa). If your telomers are long enough, you are not afflicted at all, your body adopts normal immunity to the virus and defeats it.

Weaknesses in this: individual variation, individual immunity. There cannot be a hard limit that says "Until 18 years, 0 months, 0 days you'll be fine... above that - one day later - it is 100% mortality".

But allowing some fuzziness in the outcome - such as a few kids die and a few adults survive - this could perhaps be sufficiently credible. To increase the credibility and avoid nosey questions - especially since cells replicate at wildly different rates in the body - you can have the virus target a specific organ, like the brain (compare to the movie Contagion) or the heart.

Failed "experiments to prolong life"

This question reminds me of an old Star Trek: The Original Series episode named Miri:

The Enterprise receives an old style SOS signal and finds on arrival a planet that is virtually identical to Earth. Kirk, Spock, McCoy and Yeoman Rand beam down to the planet only to find that it is inhabited solely by children. Kirk befriends one of the older children, Miri, but they soon learn that experiments to prolong life killed all of the adults and that the children will also die when they reach puberty. They also learn that the children are in fact, very old. Soon, the landing party contracts the virus and has seven days to find a cure.

Star Trek Miri (TV Episode 1966) - IMDb

One of the nice things about this scenario is you don't have to explain very much about why or how.

If you cover what happened from an in-universe point of view, there isn't going to be much detail - none of the survivors will be equipped to understand what happened on anything but a very superficial level.

I can think of two examples of this approach which I really enjoyed.

The Girl Who Owned a City took the viral approach, killing everyone older than 12. The protagonist, Lisa Nelson, is far too preoccupied with the business of surviving and caring for first her younger brother, and later her citizens, to spend much time pondering how everyone died.

A Tunnel in the Sky took a slightly different tack, the adults weren't dead, the children were stranded. It's the same idea, in that the children have no means of determining what went wrong. They have no access to the relevant equipment, nor the expertise to use it if they had.

As long as it's internally consistent, lack of information doesn't have to be a bad thing. It really falls apart if there's someone who really should know what happens, so be careful of that pitfall.

The 1918 World Flu pandemic did almost exactly what you are asking for.

An unusual feature of this pandemic was that it mostly killed young adults. In 1918–1919, 99% of pandemic influenza deaths in the US occurred in people under 65, and nearly half in young adults 20 to 40 years old. In 1920 the mortality rate among people under 65 had decreased six-fold to half the mortality rate of people over 65, but still 92% of deaths occurred in people under 65.

But why would it kill young, healthy people, and not babies and the elderly?

Modern analysis has shown the virus to be particularly deadly because it triggers a cytokine storm, which ravages the stronger immune system of young adults.

People without strong immune systems didn't have as strong a response, and thereby avoided the cytokine storm.

Great ideas! Pretty heavy on diseases and biological explanations, so how about a few that are different?

1. A society chooses a handful of children -- perhaps children of powerful politicians, athletes, scientists, etc -- to protect from a massive war or alien onslaught. Perhaps they are accompanied by some adults as they head to the safe place but the adults end up dying in a rear-guard action to save the kids. The safe place might be extremely small and deep underground, or could involve being launched into space in a ship that will take several years to get back to earth.

2. An alien invasion where the aliens kill anyone who resists, then they take all adults off-world for slave labor. The aliens view humans as we might view wild animals, so they leave the young ones and expect that if they come by in a few centuries they'll be able to harvest a new crop of slaves. (Having decapitated the earth's scientific apparatus, the survivors will fall back into the stone age fairly rapidly, though with lots of strange and failing technology around.)

3. An alien invasion where the aliens like to eat humans, but only ripe humans.

4. OK, one last medical thing: some new manufacturing process releases a ton of nanoparticles into the atmosphere and these particles are extremely toxic and kill most everyone whose cells have aged past a certain point. Younger children have less-aged cells and are able to withstand the cellular damage that the rest of the population cannot. This wouldn't precisely be a 17-18 cutoff, but might work down to what we view as the minimal viable age of self-survival and then the story begins however many years later such that the oldest survivors are 18.

There's actually a book series that deals with exactly this scenario called "The Fire-Us Trilogy" and starts with The Kindling.

It deals with a younger group of kids after a virus (the Fire-Us, as the characters call it) wipes out all the adults, and does a nice mash-up of Fallout and Lord of the Flies.

The actual mechanism of the virus is kind of spoiler-y, but if I recall correctly, it has to do with the amount of sex hormones in the body. The higher your levels of Testosterone and Estrogen, the more likely you were to be affected and later die from the virus. This meant that children who have relatively lower levels of testosterone and estrogen were unaffected while teens and adults were decimated.

There really isn't a great biological trigger - puberty is too young, end of puberty a bit too high (some changes in some people lasting into the 20s, not ideal if 18 has to be the cutoff - not to mention some medical conditions might apply).

But, you might find environmental triggers. So, what do adults do that under-18s don't? ... well, nothing. All the age-related activities are restrictions, and therefore optional, and adults who did opt out will survive, skewing your population. On the other hand, what do under-18s do that adult's don't? Hah - maybe school works.

If school is compulsory (like in our society), then 18 would be about the age of graduation. I recall we had a daily fluoride rinse program in our school - not strictly relevant, but there may be some other health-based program running at the right time that kids had to attend, that adults mostly were not bothering with. Maybe vitamins? Some kind of exposure from school buildings or chemistry lessons? Something that happened when the whole apocalypse got started - like barricading themselves in and avoiding the whatever? It doesn't have to be much, just enough to make it survivable.

Of course, this wouldn't be a perfect measure. There would be those younger than 18 who died because they had graduated, or dropped out, or were absent at the wrong times. There might be a few older than 18 who survived - maybe late graduating, or happened to be doing the health-whatever. They could be few enough to not be influential on your society, or perhaps they were few enough that they did wrestle with the kids for control...and lost, but they would be there.

Another option would be to tweak your society slightly, to make some event compulsory at or after 18. Maybe a citizen oath or registration or health check or something, that they have one year from their 18th birthday to accomplish - and some exposure or side effect from whatever happens during that is what makes the adults susceptible. You would loose about half of your 18's, since some would have done it and some not (or your could just say 19, I guess). You might also have a few off-the-grid types who resisted, but again the number could be small and for whatever reason not influential. Or maybe have all adults required to be present in their local coming-to-adult citizenship meeting each year (or season, etc) to affect everyone at once. Again, a few adults might be missed, but they don't have to be enough to be influential.

Or to go in a completely different direction

A lot of illnesses may have different strains or adaptions going on, and we might end up with a population exposure difference. One option might be that, if a variation of the illness (or a related one, or even a different one) was sweeping the schools just before the apocalypse-illness got started, maybe they sent all the kids they could into communal quarantine - especially once they realized kids were surviving where adults died, and maybe they didn't know it was strains rather than age-related protection.

A fair chunk of post-pubescent kids might survive just from not being exposed to the adult version, and/or getting protection from catching a lesser strain (like cowpox vs smallpox) - which showed up in schools and spread like mad. Maybe with a plot point that adults didn't catch that lesser strain because of some prior vaccination or medication or something (since phased out), which the apocalypse-virus was just different enough to slip by. Maybe it was a few quarantines of teens for an unrelated illness, with prepubescent kids surviving the apocalypse-illness due to age-related expression, with only whatever teens were in quarantine surviving due to not catching it, but afterwards taking leadership roles (and generally being visible) because the littler kids needed them.

Here's a keep-it-simple answer: An alien race decides to do a "reset" of the planet - killing off all the adults for some offense the planet has committed.

I don't believe any explanation is necessary how their advanced alien tech is able to know who is an adult and who is not. In fact, no explanation is necessary for even why the aliens decided they needed to do this. Speculation on this question could be an interesting weave in the story.

If you must have a scienific explanation for how they know the adults from the children, have a look at https://www.thenakedscientists.com/forum/index.php?topic=31107.0. Basically the important part is that we know where we can find cells in the brain that are as old as the individual and we can assign an age to those cells using some variation of carbon-dating.

Reminds me of a short horror-story I wrote once where all the meat-eaters get mad-cow disease and the world is taken over by ... (scary music here) ... vegetarians.

One government developed a super weapon that can kill specific targets based on their DNA (with the intention of being able to poison a water supply and only kill the intended victim).

Now combine that idea with a country that logs the DNA of all citizens of voting age (so they can be certain that everyone only voted once, because their vote is tied to their genetic signature) into a central database.

Somehow the database gets linked up to the weapon (which is constantly flowing into the water supply) thus killing everyone whose DNA was in the database, i.e. everyone of voting age.

Only downside is that it could only really affect one generation unless the future generations continue adding their DNA to the voting system and don't make the connection.

Herpes. Most people who have it don't even know it so it spreads. People with weaker immunity have cold sores and stuff. This idea would kill off some kids but not the bulk (vaginal births and kissing relatives expose youngin's). There's been research over the last few years that might suggest a possible link between herpes and Alzheimer's. It could mutate and be fast acting/spreading but essentially Alzheimer's only happens with some age... the early onset could continuously getting earlier but maybe a developing brain wouldn't be effected until early 20's. This would need a generation or two to happen. Shingles is in the same family as herpes. Chickenpox, too, but kids these days get vaccinated.

Think Small

Aging (at the cellular level) has been shown to be strongly linked to length of telomerase strands which cap the end of the DNA sequence in living creatures. Interestingly enough, these strands are species specific.

Should there be a genetic "virus" or similar effect which acts lethally only upon those whose telomerase strands are over a certain length in a given species, you will find that the vast majority of individuals over a given age of said species will succumb to the lethal effects of this most unusual plague.

Well, in order for some sort of disease to target only those over a certain age, you would have to examine the physical changes that occur around the age that you're speaking of. The first thing that comes to my mind is the fusion of growth plates (which stops us from growing forever). For males this usually happens somewhere between the age of 16-21, most commonly around 18-19.(Taken from https://www.zocdoc.com/answers/21489/when-do-most-males-growth-plates-close) I'm not entirely sure how a disease would trigger based on such a physical change, but its a possible cause. Hope this helps!

Vaccination would seem to be your only option. It would not save all children or kill all adults, but you could get into the very high percentages quite easily. For example, an update to the standard MMR jab that included a polio vaccine could become the new normal. 15 to 20 years after it is introduced a new fatal strain of polio wipes out everyone not vaccinated. The result would be that 90% or more of all children would survive the initial infection, but 99.9% or more of all adults would not.

Perhaps a natural phenomenon or a world war kind of deal where something is dispersed (chemical, viral, etc.). Or maybe something happens in space like some sort of rays that the earth passes through or a burst of something that envelopes the planet, etc. That way you can account for everyone on the planet and not miss certain populations. Everyone on Earth is affected and is slowly dying. They can still reproduce, etc., but within 18 years everyone that lived on the planet through the event dies, leaving only their offspring.

Amy's answer is the simplest. I would prefer to upvote or comment on her answer but in lieu of that...

My take is that there is an undetected global pandemic for a period of years that infects everyone on Earth without harming anyone or showing any symptoms. After a few years (once everyone is infected), the virus or whatever goes dormant for 18 years before suddenly killing all those infected. Only those 18 or younger would be left alive.

Simply have a pandemic infection 18 years ago that made everyone infected (which was almost everyone) susceptible to some new scourge.

Among other things, this allows one to explain how a few "oldies" (as many as you want) survive -- they were not infected originally.

Don't target an age. Target a life experience.

Let's see... I'm writing this answer in the year 2017.

In 1985, after Halley's comet flew by, the solar light interacted with the comet in an unexpected way. The reflected light beams had some properties that ionized Earth's atmosphere in a way that caused a chain reaction. The effect took over ten years to develop, but once the catastrophic result occurred, the effect was nearly instantaneous and planet wide. Everyone on the planet breathed in the toxin.

However, those who weren't yet born gained an immunity during pregnancy.

In 2017, an upgrade to the satellites used by GPS caused a fatal cancer in anyone who wasn't immunized. Unfortunately, the people who caused that problem are now dead, and nobody quite figured out exactly what happened in 2017.

You'll probably want to change that significantly. (Blame the Chernobyl explosion, or any other one-time event that hasn't happened in the last 18 years. The beauty of Halley's comet coming by Earth is that you have a need to find a fix to the problem in the next 76-18=58 years, or else the catastrophe might repeat.) The point of the example isn't as much to be your ideal approach, but just to show the general technique: rather than target age, you might want to target specific years. (The precise cause can be different, yet you end up getting the same desired result.) What happened on your planet at a specific year, but then didn't happen again afterwards?

Note one downside: It makes your writing timely. That is, if you actually based things on Earth's dates, your writing will seem less applicable 8 years from now, unless you use relative dates (20 years ago) or some other dating system (a non-Earthen calendar).

What about explosive ID cards? With some handwaving, you could establish that in your world, all adults were issued one. If you have trouble coming up with an explanation for an explosive card, what about hack/malware wiping adults through an ID chip being routinely implanted at some appropriate age?