How can a sim age up every year on the same day?

@HillyBeth Ah ok. Yes, you would need a mod that allows you to tweak each life stage individually. The regular lifespan stages are not the same as each other. The baby stage is very short, the toddler stage is longer but still much shorter than young adult, etc.

The simplest solution is probably @babajayne 's suggestion, turning aging off and using the cake on the proper day, or bassinet for babies (you can still leave aging on for NPCs if you want to, that's a separate setting.)