Two things about it mainly. The first is that despite it being an "adult" thing, nature says we're ready at 13(what with puberty and all). Is nature just trying to get the awkwardness out of the early or.... is there something else I'm missing.
The other thing is STDs(Or STIs if you insist). Obviously it's risky if you do it with someone that already has one but, how did it start in the first place?Two clean people shouldn't end up infected just by doing what nature wants you to do to reproduce (minus one crucial part if you just want sex). Or did someone just do it wrong, tried to put slot A in slot C instead of B or D and that screwed things up for everyone?
Just a small example of the random stuff that pops in my head.