Without being able to prove damages, I'd say no.

There's tons of research on the damage caused by childhood sexual trauma. In many cases that damage doesn't manifest for years after the fact, meaning that proving specific harm to the child within the statute of limitations for a crime could be pretty much impossible.

Then again, I see little reason why 12 year olds aren't competent enough to decide for themselves if they're comfortable being with a 40 year old.

Really? Have you been around a lot of 12 year olds? They will eat nothing but candy and pizza if left to their own devices. They will stay up too late and play video games all day and never go to school. 12 year olds are not known for their ability to make good life choices. Which is obviously because they lack maturity, wisdom, experience, fully formed brains, etc.

There is also tremendous potential for manipulation and abuse when you're talking about an adult and a child. Finding a true, free, knowing, voluntary consent in such a situation is extraordinarily unlikely.

But regardless, I do recognize that there exists an age (less than 12 imo) when children would need to defer to parental discretion

OK, so if a parent, in his discretion, thinks its OK to let a 40 year old fuck his 8 year old, is that OK, so long as the parent signs off?

In which case, there would need to be proof of something being damaging, or having an overwhelming probability of being damaging in the future [adult life].

There is absolutely proof of such an overwhelming possibility when you're talking about childhood sexual abuse.

studies show between 15% and 49% adults aren't symptomatic after sexual encounters with adults as children.

Please do tell. The fact that somebody isn't symptomatic at the time of a study 20 years after they were abused does not mean that the abuse was OK. Psychological conditions are not constantly symptomatic.