Something is corrupt in your game. I have had this happen. You need to delete all the sims that wont age in your game or the problem you are having will not go away. It will effect you whole town. Mess up your family tree etc.
Here is how you get rid of it because it will effect your whole town. Any sim in your town that wont age up you have to delete. You may also need to move your sims to another house. I knew which sims (non active house) was causing the problem. deleted those children but the sim in my active household stll would not age up so I had to delete that sim too. The house that was causing the problem I moved those sims to a diffrent house and problem was gone. Moving some of my sims out of active household while pregnant is what caused it. It happened more than once so I figured it out. I did the move out option in game and let those sims use some of my active household simolens to buy them a new house, Did not move them in edit town if that helps