What do you mean they get younger? Is that even possible?
Is it happening to all of your sims (Adults, young adults, teenagers) or not?
I found here something that might help you: Using MasterController's age up feature works most of the time to fix the issue.
Note: MasterController can cause a bug while using the age up feature where you will have a freaky looking Sim (a child that aged to teen but game still thinks it's a child) to fix this issue use the MC CAS editing feature, make sure its set to right age, exit and if needed resetsim.