From what I understand this is a known bug. However the work arounds are kind of unclear if they really work or not. Here are a few suggested things to try:
-- Evict the family and move them back in to either their house or another house.
-- Take the Sim into CAS and age them up that way. - On the day of their birthday.
-- Go back to a previous save.
I'm not entirely sure how well any of these suggestions will work, other than going back to a previous save fixes some problems. As I haven't experienced this bug.
I hope this helps,
MrsFlynn