i'm 20... too young to care.
and to be honest, i don't think i'll ever truly care.
as long as i can walk on my own, hear half of what's said to me, and NOT go in my pants, i'll be happy.
interestingly enough, i seem to look old enough that waiters/bartenders automatically hand me a beer/wine list and don't bother carding me if i have a few day's stubble.