Heh. When I was in tenth grade I was dating a French gal, and the few times I had dinner at her parent's house, there was wine with dinner, including us.
In the 18-21 age range I was trained in weapons, got a TS security clearance, worked on B-52Fs, B-58s, and one broken SR-71. Oh, yeah, I also got married.
It wasn't 'til I was older, though, that they shipped me off to a war zone - but not THAT much older.
Maturity is not necessarily related to age. I've have met youngsters who were mature well beyond their tender ages, and I have me middle aged adults that I would not trust to do anything reliably.
After all that is said, Heinlein got it right, in my opinion... "Beings must never be judged by categories, but only as individuals."