Whenever a character is healed magically, there is a percentage chance to age one year equal to the number of HP so healed, minus the character's constitution modifier. For example, if a character with 10 (average) constitution is magically healed 6 HP, there is a 6 percent chance of ageing. If a character had a +1 constitution modifier, the same healing would result in a 5 percent chance of aging.
|Gandalf is actually a 22 year old wizard|
who overdid it at a frat party.
Another thought is what happens if a magic user overdoes it? What if things go down and a wizard has to turn it all the way up to 11 and cast lots of powerful spells or harness vast amounts of magical energy, more than he's accustomed to or should be doing? Well, if using magic on a normal basis causes him, her, or it to age about a year per spell, then this could age rapidly like Nazi from Indiana Jones and the Last Crusade.
Wizardry, much like pimpin', ain't easy.
Edit: Another thought just occurred to me: What if items caused aging either passively through mere contact/ownership or actively through use? Example: A weapon that lets you fight like a total badass, but ages you weeks, months, or even years the longer you use it. I dig this because it forces the owner of a such a weapon to think their way out of situations and not merely fight.
Picture via Fanpop.