Since you un-installed University, all of your Young Adult sims aged up to Adult. If you're looking for a cheat-free way to do it, there is none. You'll probably have to end up aging the sims down to teen once again, then sending them back to their freshman year of college. You can either do that by typing in
agesimscheat on (adds "Set Age" to the Interaction menu and you can click on a sim to set them to any age group you want) or you can Download the InSIMenator and use it's Temporal Adjustor to change the age of the sim.
That, or use whatever aging mod that you prefer. Those are two ways that I use sometimes. :angel:
