I vote teens. I'm currently playing my first teens in TS4 and I agree with others who say they need to look more distinct and have more unique actions. I'll be honest, I don't have any ideas on what that would mean. But I'm sure between the developers and the community, there are fantastic ideas out there.
I kind of like how babies are now. Nice and simple. Like the calm before the storm of toddlers. It helps ease into balancing caring for the offspring and regular life with work and stuff. For the people who want more for babies, I guess I can sympathize but I don't see it as an important change in comparison to other things. But you can just go ahead and age them up and launch yourself straight into toddler wrangling.