Hi Steak2000,
Welcome to the forum. Unfortunately changing a sims age to make them younger in CAS removes all the trait rewards earned from completing aspirations. Even worse the aspirations still register as 'completed' in-game, so you can't use the 'aspirations.complete_current_milestone' cheat to get them back. I don't know of a cheat that removes a completed aspiration and I don't know of a cheat that can give a sim an already completed aspiration the corresponding reward trait. I did this with one of my sims and ended up just playing him anyway even though he had lost all of his super cool reward traits. Luckily I still have an 'uncorrupted' version of him in my library.
I guess basically, unless you do find a cheat to reverse the damage all you can do is play your sim how he is or revert to an earlier save. If you do find a cheat to reverse the damage could you post it here please. I'd love to know if this is fixable somehow!