I've not run into this particular bug just yet, but here are some ideas I can come up with:
Go to the Salon and get a makeover/tattoo, try some ambrosia or plastic surgery. See if other methods of anti-aging help refresh the age bar and get it ticking again!
Try resetting every sim it's effecting? Probably been tried but I'm just going to throw it out there.
Even though you're not allowed to yet, try a test save to see if moving them to a new town fixes the issue.
If the only thing that seems to work breaks the rules (like moving towns ahead of schedule) you can do the tedious method:
Look at how long each stage is supposed to last and keep a mark on it and age them up with a cake.
Or another plan - age them up with a cake/age of instant potion and see if that refreshes their age bar. If nothing else works, the counting method should help - though it's tedious. Let me know if you discover anything!