Game Help Talk:Babies

The minute the notice appears that the baby will age up in another day, you can buy a birthday cake. Click on it and the option "Bring [baby name] to Cake" will appear. It's a lot easier than the parents or a sibling constantly trying to Help With Birthday...which for me has taken as long as an entire Sim day!

And make sure that the baby has had its diaper changed recently...if the baby dirties its diaper when trying to transition, the aging up won't happen. What you do is change the diaper, buy another cake, and try again. (Or just make sure you've saved the game, back out, and start over.)