"Larger issue - what weight does any argument have pertaining to "mental capacity" or the like if the sex itself is not causing them any harm?"
that's my point.
I mean, the person is going to have sex not lead a war
no one needs to have a fully developed brain to enjoy a few minutes of pleasure
and the brain only develops fully at 25, not at 18.
so, unless the argument is 25 years old, it is totally invalid
what's the difference between a 17-year-old and an 18-year-old?