I confess to being a bit baffled by the different life stages in The Sims 3.  In the Sims 2 base game you had six life stages: baby, toddler, child, teen, adult, and elder.  Then of course the University expansion pack came along and added "young adult," to cover the college years.

Now for some reason Sims 3 has split up the "adult" years into "young adult" and "mature adult."  Some stages also seem to pass more quickly (particularly baby, toddler, and child) while others move more slowly.  I have a Sim in one family who just celebrated her 102nd birthday, and this is with the normal (i.e. not "epic") life span!

The strangest thing about this is that according to the Sims Wiki, the Baby, Toddler, and Child life stages each last a little bit longer in Sims 3 compared to Sims 2.  The only thing I can think is that they SEEM to go more quickly in Sims 3 because the game has been tweaked to make these stages easier. 

I haven't been able to gather any hard data, but it really seems like you don't need to feed or change babies more often, and that they spend more hours sleeping.   The same goes for toddlers, who seem practically self-sufficient.  (Of course, I still haven't been able to get a toddler potty trained, and taught to speak and walk before they grow up into children.  But that's squarely my fault, not the game's.)

What really puzzles me is the difference between Young Adult and Mature Adult.  I haven't been able to find a single difference between the two stages, although their appearance changes a little when they mature.  (My most recent Sim to age into Mature Adult was identical, except that her boobs dropped fractionally.)

I suspect that these two categories were built into the game from the beginning, so that they could be used later in a University-style expansion pack.  That's the only reason I can come up with for the fact that they exist now. 

The Teen and Elder stages seem essentially the same as in Sims 2.  Elderly people are stooped and move slowly, while teenagers never want to do their homework.  Teens (like children) still have curfew, and apparently having the police bring you home at night is very embarrassing!

Interestingly, so far what I have observed is that the higher your Sim's Lifetime Happiness, the longer they will live until dying of old age.  This is equivalent to the lifespan bonus which having a high Mood granted in Sims 2.  Just as an example, my 102 day-old Sim has accumulated a ridiculous number of Lifetime Happiness points.  I've cashed out about 20,000 points so far, and she has accumulated 34,000 more.  (I'm saving up for the food replicator.) 

What can I say?  The Clean Freak trait is really handy.  All of her wishes have been cleaning [3]-related, which she tends to fulfill all on her own.  And none of her wishes are ever crazy or unrealistic - they're all along the lines of "scrub the bathtub" and "wash the dishes."