My primary Sims, Adam and Lisa, are immortal young adults. I designed both of them, and they are there as my favourites to experience the game. Everything else is a distraction or an addition, and ultimately they might be serious about raising a family. I figure they are about 100 to 150 days old right now. I'm not very fast.
I made only them immortal though. When I have a child or teen, I stretch it out to about 20-30 days, because that is their purpose to me. To experience a different life state, so no need to rush. I have no interest in more adults. This allows me to keep story progression going, I enjoy the slow dynamics of it. Besides, I'd never get my reaper snowmen if nothing ever died on my property (I have a ghost cat that likes to visit me at night now, but without the obligation of feeding it so it's pretty awesome).