@oliver I'm more impressed than anything that you're so young (I mean, wow) but isn't it pretty normal for RP'ers to play a character that's a few years older/younger than their real age? So playing a high schooler should be rather doable, at least in my opinion. You're going to be pretty limited if you just stick to playing thirteen-and-under characters -- I recommend expanding your horizons :)
This. It's like saying "I can't play a female, because I'm a male".