I like them all, though my YA's definitely seem to get more love. Mostly because that's the stage where they get married, can have a "real" career and start pumping out babies. I have doubled all the ages but baby to get more time with my sims, and I'm always maxing out toddler skills...not always child skills, mostly because I don't want samey same kids. But I do try to max the skill that leads to adult skills that they're going to need so that they get a headstart on them. And I quite enjoy getting Parenthood traits, but I don't necessarily go out of my way to max them all, again, samey same. Teens generally get a headstart on their aspiration, or complete the first one (if they can) and move onto another one. I'm a bit of a trait hoarder, so my sims start collecting reward traits as early as possible.
Also, as far as school goes - those with an A I believe start higher in the career. But I could be mixing that up with something else, I haven't had enough coffee yet.
