Sounds most like a conflict with something else which affects or is capable of affecting aging. Some (but by no means all) likely culprits: Inteen or hacks involving allowing vampire or zombie aging.
Would recommend using Paladin's HCDU first:
http://208.109.175.73/Sims_2_HCDU.htmin order to see what possible (but not necessarily malignant) conflicts you may have, and pulling those out of the game to check if that rectifies the problem. If not, then you'll have to do a standard hunt and check of custom content to find what is conflicting or what may be installed which is not compatible with your particular installed combination of EPs/SPs.
If the direct link above doesn't work for you, then go to:
http://simwardrobe.com/And click Sims 2 and then Programs and find the Hack Conflict Detection Utility.