1- You can activate testingcheatsenabled true, then shift click on your sim and choose Edit in CAS. From there you can age down your sim. After that, remember to disable the testing cheats (testingcheatsenabled false).
I did that once to age down to teen one human turned imaginary friend who had aged-up to adult before his “owner”. I had no problems in that save and he aged-up again normally, had kids, jobs, no problem. Except that he had a random boarding school diploma already, but I was able to enroll him in the school again by sending him there, though he didn’t get a second high school diploma later.
To avoid problems, just make sure your YA is not employed in a job that teens can’t have and is not pregnant. And backup your save as problems may happen as igazor said.
