I agree with what you're saying.
if I understand correctly then you don't think age is the problem, but the level on which she thinks is. her knowledge of the world so to speak.
problem is that these young people need to be protected by law and as certain people are earlier mature/grown up then others it's very hard to tell when each girl is considered mature. so they drew a line. in some countries thats around the age of 18, in others it's 21, in others it's earlier or later.
some girls however, reach their maturity earlier (or later) which is why we see young people naked on cams, hear kids aged 13 or 14 talk about sex like they've been doing it for years, and some actually have... and we can't stop that.
I lost my virginity at the age of 17 but I know men and women who lost it way before me. like age 12 and 13. that may sound sick to you, but these people where actually very mature at that age. more mature then I am right now (aged 24

).
the world forms people and some people take on their shape earlier as others. those people are more likely to mature earlier.
just my opinion