So, I have used boolprop testingcheatsenabled true to age a sim from child up to toddler and up to child. I think that this may be cause of the bug.
And now anywhere I play, the first-moved sim or the oldest shows an "1 day left before birthday" pop-up every day at 6pm, even through it's not even close to their birthday, and also they stop aging, but the rest of the household ages. It happens only to the first moved in/oldest sim in the household.
Also, I've tried moving a sim like this in another household, but it kept this bug.
What can I do to get rid of it? It's really very annoying...

Do you have an aging mod installed?

If you use InTeenimator, try spawning the affected sim's biological clock and then putting it away again.

I have the "Real life" mod I found there on MTS which I don't use anymore as I can make the lifespan as long as I want. I might try to delete that mod to see what happens.
Oh, and I forgot...I was playing at the Curious household in Strangetown (and I aged Pascal's alien child) and Vidcund was the first affected by this, he was also pregnant at the time. When his baby was born he was misteriously aged to a teen with one day until he becomes an Adult. I haven't saved and terminated his pregnancy with Inteenimater, but had the same problem.

The problem with Vidcund turning into a Teen is a common Inteen one, solved by spawning the biological clock. Nysha's right - try that first for any of these problems as long as you have Inteen.
And yes, if you're not using a mod, delete it. The game's complicated enough without things you don't use lying around altering the code.

Gosh Nysha thank you so much! I've did that and my game is all normal again 😂
I didn't know about this Inteen problem before and I thought that I did a mistake messing with testingcheats to age a sim but I'm glad it isn't that
And I also removed that mod, don't need it anymore with sim blender installed. Now I'll go solve this problem at every household
Thank you very much!