I think enough of the human race had died at that point to where people willing and able to become teachers ( especially those who had helped save the remnants of the human race ) She was mature enough through her psychologically sped up life to flourish in a teaching environment as well as a combat role earlier on. It's Post Apoc... if there are 16 year olds with college degrees nowadays in the real world, who says a girl on the brink of human extinction couldn't become more than a sex symbol?
Amazing! Maybe she was young, but the Japanise have a wild affinity for little girls, thus anime is full of young ladies barely dressed, what does that say for those of us who love them? Kinda sad really, but they are so fun to watch!