LOL!
The problem is that they took away the option to just age up. So if you don't want to wait around until 8/9 pm of the last day and have it interrupt whatever they're doing, then you have to use a cake. I see the point though, how then would you distinguish when you DO want a party with everyone in attendance. Although, people who have chosen to go to bed wouldn't typically wake up to go watch the candles being blown out...
You could always use the cheats to age up. Can't remember which setting enables it, probably debuginteractions, but there is one to "Trigger Age Transition", I have used this from time to time to age sims when I have aging turned off, and want to age a couple of sims, or I want to simply move them along faster, without the cake/party hassle.