[CURRENT ISSUE] Aging does not fully revert when quitting without saving

I have reproduced this in my game by aging a child into a teen and it happened every time.

For some extra context, when quitting without saving, the game correctly reset the Sim's stats (age info was set back to child, they got the Grow Up aspiration back, and were technically considered a child, etc.); this info is stored in the Neighborhood package, which seems to be handled correctly depending on whether I saved or not.

However, the Sim's 3D data, like mentioned by the OP, was saved even when saving and quitting. This data is stored in the Sim's character file, and reloading causes the same issue as in OP's screenshot.

For now, I highly recommend keeping whatever appearance they have once they age up. You can always go to a clothing store and purchase whatever you'd like them to wear. I have been able to age up the bugged Sim the second time which fixed their appearance, but I cannot guarantee there won't be any long-term oddities.