To get gradual aging they'd need some sort of distinct "year" even if only internally (after X Sim days, make Y group of changes) They'd also need dynamic rigging and scaling so that animations and clothing would be usable through these changes. While the technology exists, I don't know that it exists at a level that's practical for a video game, as opposed to CG animation.
Then you'd also want to consider that people don't actually age and develop at as universal a rate as the school books say we do... plus the issue of different height adults, and that sometimes teens are actually taller... (I was mistaken for a pre-teen as late as my early 20's, and my daughter's experiencing the same)
I know it's just a game so it doesn't have to be that realistic, but if we're hypothesizing on gradual aging, then we should look at all the variables people would hypothetically want for that gradual aging in the pursuit of it being a life simulator.
