I guess young adults are my winner. Usually, if a Sim is one I want to keep playing and develop, I age freeze them once they hit YA, but I also really like raising new sims as well, which conflicts with my desire to control as close to two sims as possible (I'm a micromanager, even if I'm not productive. 4 sims is my limit).
I really like going through the 'story' of my sims rising up in the world, and I'll eventually become disinterested once they stop developing as a person, or sometimes, once a more interesting story presents itself to me.