I find it interesting that a lot of the guesses here end the adult stage at 55 and then jump to 65 for elder. My real life age is between those two, so what am I? o.O lol Anyway, I understand why it's perceived in this way though, I see that as well. The game does depict quite a jarring jump between these two age groups.
The jumps between baby to toddler to child to teen is too drastic in my opinion as well. The only transition that is somewhat realistic is teen to young adult to adult.
This is how I see the age groups as depicted in the game:
Babies - 0 to 3 mos. I'd say older, but they never leave the bassinets
Toddlers - 1 to 3 yrs. The average baby begins walking/toddling between 1 yr. to 18 mos.
Child - 8 to 12 yrs,
Teens - 16 to 19 yrs. IMO teens look way too old in this game to be 13-15 yrs. I do know some teens physically mature earlier though. The opposite is true as well. Some young adults look like teenagers well into their 20s. *shrugs*
Young Adult - 20 to 35 yrs.
Adult - 40 to 55 yrs.
Elder - 75 to ? Here in the US, people start getting 'Senior' discounts at age 62. I would've started this at 62, but that's not how the game makes elders look. They're MUCH older looking than that in my opinion.
Having said all that, I really don't have a big problem with the age group transitions. I'm having fun in my game despite the way it is now. It's too late at this point in my opinion to add anymore anyway. Mostly because I feel it would involve a complete overhaul of the game with all the different heights and animations that would be needed. I'm guessing this would take too much time and expense for a game this far into development.
If they were to attempt a major overhaul of any kind I'd much rather the focus was on improving emotions, adding memories, likes/dislikes, more traits, etc. to give our Sims better individualized personalities.
Happy Simming! <3