IF YOU TRIED ALL THESE SUGGESTIONS AND IT STILL DOESN'T WORK AND IF YOU HAVE MODS
I had the seem problem. However, after I tried all the thing you guys are suggesting, it still didn't work. So I assume it was a conflict with one of my mods (which it was). For me it was the Inteen Pregnancy + Mod that I got from this site <Link removed since the mods there raise the rating of the game, hence posting them here is against the TOS> . (It could be a different mod for you.) As soon as I removed that mod everything work like it suppose to. I could age up babies again.
These are the steps that I took to solve the problem for me. Now I know most people that have mods probably know how to go through the mods, but just in case you don't, I will tell you the steps I went through.
Step 1: Removed all your mods from the mod folder and temporally put them in a folder on your desktop.
Step 2: Try the game. See is the game is still not working. (As soon as I did this everything work fine.) If it still doesn't work, then you know there is something wrong with the game setting and it's not conflicting because of a mod.
Step 3: If it does work properly, put all your mods back into the mod folder.
Step 4: Temporally remove and put in a folder on your desktop ALL mods that deals with pregnancy and/or babies.
Step 5: Try the game again. See if the game is working.
Step 6: If it does work, place one of the pregnancy and/or baby mods back into the mod folder. Only One!
Step 7: If it continues to work, then you know it's not that mod that's conflicting. So you can leave that mod in the mod folder.
REPEAT STEP 6 AND 7 UNTIL YOU FIND THE MOD THAT IS CAUSING YOU NOT TO BE ABLE TO AGE UP BABIES
Step 8: Once you find that mod delete it and return any remaining mods back into the mod folder. However, keep in mind, that there might be two mods or more that are causing you this problem. So you might want to continue returning them one by one, just to make sure.
IF IT STILL DOESN'T WORK
If you complete Step 1 & 2 and the game work perfecting, but run into problems when completely Step 3 to 8 you might have to go through all your mods. So basically remove all your mods from the mod folder and put them temporally into the folder on your desktop. Now you can either do it one at a time or 5 at a time or 10 at a time, it doesn't matter, put them back into the mod folder. (The more you put in at a time the faster it will go, however, you have to remember which mods you put in just in case one of the ten is the one causing you the problem. I find it easier to do 5 at a time) After you put some of the mods back in try the game. See if it works. If is does, continue to put more mods into the folder. Keep doing that until the game stops working properly. When it doesn't you know on of the mods you just put in is conflicting. So you have to go through the mods you just put in one by one to find the conflicting one. Once you find it, delete it. You can now continue to go through the rest of your mods or you can try your game to see if it now works properly.
Hopes this helps. Good Luck My Fellow Simmers.
Morgana Grey