The difference between an ADHD brain and a normal adult brain is similarly unquantifiable (
in terms of general abilities) as the difference between a minor's brain and an adults. Neurologically, an ADHD adult has synaptically pruned, though, I guess.
though evidence suggests this isn't true of autistics, and while I'm not sure whether the experiment tested high or low functioning autistic brains, I suspect high-functioning autistic brains to have somewhat similar traits.
Anyway, what with the whole brain damage example being there too I think it's clear that neurobiology isn't enough to argue a group can't consent.
The power imbalance can be mitigated, possibly with legal aid if you're worried about it that much (for instance, there could be signed legal documents to the effect of "if this child comes forward, we're arresting you without trial"--not perfect, really, but combine that with actions on the part of the adult that might allowing the minor to say no to a bunch of other things first to get them comfortable with it and it becomes pretty damn close).
Also, this raises another point, too--the power imbalance is pretty much eliminated when dealing with a prostitute/client relationship, where the adult is the prostitute. So, if we assume that power imbalance is what makes sex nonconsensual, wouldn't that mean that a minor would
not have been raped if they had sex with a prostitute?