I figured out how to fix the birthday glitch. There are actually two possible causes. One is addressed in the video and the other is addressed in the comments section.
For anybody on mobile who can’t see the alternate solution:
NOTE: For some people, the glitch has a different cause. Before your sim ages up, make sure that their interaction queue is empty. If they have an interaction in their queue while they’re aging up (like when sims multitask) the sim will glitch out. The solution is to use a cheat to reset your sim.
Oh, thanks for posting here, I had this glitch too, and it's good to know that there is a solution without mods. I lost my patience after the second failure, so I've used mccc for aging up the birthday child.
Comments
My Plumbob is GREEN today
https://littlemssam.tumblr.com/post/175412067973/random-bug-fixes-a-collection-of-random-small-bug