Today I had some time to play so I turned off mods and tested with a brand new save. The first sim in the household to age up was a toddler who aged up as soon as he woke in the morning, 5 AM.
Yes, it was marked on the calendar for that day along with another birthday (I didn't keep playing once I saw the first sim was going to age up, the other sim with a birthday on that day was still asleep but didn't have an age up action in the queue). When I returned to my normal game (with mods turned back on) the next sim to have a birthday didn't show this problem.
I have a feeling that it may be the first sim to age up after changing lifespan settings. On my normal game I had used MC Command Centre to make a custom lifespan and the first sim with a birthday after that aged up early in the morning on the no mods game I switched to short lifespan and the first sim who had a birthday aged up in the early morning, the next one to age up (even though the birthday was on the same day) didn't seem to be affected.
