I just played for the first time since the patch/upgrade/new download/not sure what it was. The first thing I happened to do was visit someone's house who had an infant.
I clicked on the bassinet, and, sure enough, there it was, the "Age Up" option.
Personally, I love this new function. It will help with story progression and also with children of your sims that don't live with them because the mother is a single woman living elsewhere.
I also like playing with aging disabled so being able to age up unplayed infants is an invaluable tool.