Sometimes life gets hectic and your forget to check your sims age. Then suddenly they age up on you because you missed the notification that it was their birthday! For shame. Now your sim is sad.
I found this to be annoying because sometimes I just completely forget. So I disabled the sad moodlet that they get if you do not throw them a party or use a cake to age them up on their birthday.
BGC!
Edit 07/31/2022: Updated for patch 1.90!