I don’t think it’s supposed to be exactly midnight when they auto age up? For me, it’s sometimes a day or so after the birthday, like it’s trying to give you as much chance as possible to avoid the sad missed birthday moodlet - I did an experiment once, aging one twin up when it said it was their birthday and leaving the other to see when he would auto age up. I think it was more than a full sim day later, and I decided that it would mess with the ages too much to be worth skipping birthdays for storytelling purposes.
As a side note, you don’t have to eat the cake for every birthday - when I play big families, I have them blow out the candles and then immediately put candles back on the cake before anyone can take a slice and put it back in the fridge for the next birthday.
