Is there any way for natural age to be raised unnaturally? I ask because some of my characters seem to be older than they should be.
Yes. The most prominent way is from the attacks of certain kinds of the creatures in the Mire of the Damned (I think one of the ghost variants do it). If you open your character screen and check your age, unnatural aging shows with a green number (as if it had been "buffed").
Drink a Rejuvenation Potion to remove unnatural aging.