One life stage is actually enough for me, if there is no difference in gameplay. Two is good, but it would be nice if there was the "mid-life crisis" that the Sims 3 had, otherwise why have two life stages (other than to help differentiate ages when looking for a spouse, for example).
With the excellent toddlers, I'm actually realizing that there does need to be more depth with Teens. I personally do not want another "pre-teen" age (especially if the Teen stage stays as long as it currently is). I just want Teens to be more fleshed out and slightly younger looking and acting. I know the devs said they decided to focus their resources on more animations etc. rather than reducing the height but in actuality reducing the height would really help. That and adding acne, mood swings, etc. A Generations-style EP adding a school with a gym where you can have a Prom "event" with goals, bringing back slow dancing, more unique interactions and animations between parents and teens... sorry I've digressed... !!