I'm on the side of limiting the ages to those which can be given attention and be made unique. In TS4 I know many have felt the Young Adult age is the only one EA pays any attention to. Really, the only reason we have a Young Adult and not just Adult-Elder is because in TS2 they had to add a college age life stage between Teen and Adult. Then in the Sims 3 it was given more of an official status, in that you became a Young Adult whether or not you went to University.
Teens, I'd much more prefer them to be de-aged from an almost adult height to a little less than the adult stage. I'd rather that than another Pre-teen life stage.
And the only reason we have Infant and baby(newborns) is because when they created the TS4, someone, I suspect, cheaped out and decided to downgrade us to TS1 style babies. Yes, I remember those. I am afraid it may be a mistake not easily taken back, now though. Players have gotten the idea into their heads that they are two stages, and if the Sims team doesn't deliver, we may protest it.
I do like how they gave the Infants milestones and see it as being a way to fit many age stages together. Anyone who's seen a real infant develop understands how many stages that can be. If we ask for something similar in TS5, it may help some of you who feel like there's too few life stages as there are.