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

Usually I age them up myself, but this one time I would want them to age up naturally. But I have this baby now which is still a baby even five days after the 'it's its birthday today'-announcement. I play on long lifespan, but I assume they should eventually age up naturally, right? I give up the waiting now and age it up manually - as always. Happy belated birthday!