@LascusClyne
I am not sure what the difference is between those first sims and the later ones. One thing I can think of is the aging switch. I assumed that you are talking about the game generated NPCs.
Are you playing with aging on for NPCs?
Have you changed that setting between the time the first sims aged up and the later ones? That could account for them not aging.
However, I have noticed that a change seems to have happened after HSY was released. The first batch of NPC students I had were in the "Other" tab of Manage Household while the ones that came after were hidden. Hidden sims don't age for the most part. All my NPC high school students are in hidden now. Many are children of the JA skeletons for some reason. The ones I have show that aging is disabled for them so the only way to get them to age up is to move them into an active household via the Ask/Invite to Join Household option. That changes their aging, so they age like played sims.
Hope this helps.