Do you age up your babies or let them age naturally?

I almost always let them age up naturally, especially if there's a decent number of caretakers in the household (vampires make great babysitters). However if they're being particularly annoying I'll age them up at the one-day warning. The more of them there are, the longer I'll put off aging them up, though, because I've been through triplet toddlers a few times and that's not an experience which improves with repetition.