Actually, he can't, because it's perfectly fine, albeit in an "as fine as it usually is" sense.

Why do we have a system that lets girls skip a grade in school if they're intellectually precocious, but doesn't let them skip some years to be considered mature enough for sex if they're precocious in other ways?

Because it's exactly as fine as it usually is, and you have it ass-backwards. :)

...although it's perfectly fine - to the usual degree.