It's not the wait, it's the way the engine handles NPCs. It just isn't advanced enough to handle dynamic aging of NPCs.
I don't think it would be that complicated, you don't really need dynamic aging. You could have a baby, then after several game months, you get a toddler, then after a game year or two (ok, not as realistic, but I doubt many players would have several game years to wait), you can have your own bratty kid (jk - I actually like the kids in Skyrim, bratty or not).