Young adults can be a parents for teens. It is weird.

I guess it's weird depending on how you play your game. I play with aging off and really only age up sims that are babies to teenagers. Most of my sims stay young adult unless I created them as adults .. and that's usually just to get some definition and facial lines where I want them.
Lol.. if I do age up a sim to an adult and elder it's because they are on a fast track to meet Grimmy.