Brendan's got a post up about making healing magic restore hit points at the cost of aging, and perhaps making other magic or the undead cause aging as well. I've actually toyed with the idea of turning most level draining undead into aging undead, but I haven't dared go this far, but it's very tempting, especially if you can get rid of the bookkeeping.
The main problem, as I see it, is that keeping track of days of aging is going to be tedious. Plus, if you're talking about aging just a few days, it's not really going to produce much anxiety in players. I think it's better if you say something like "when rolling one or more dice to magically regain hit points, any die result of 5+ adds one year to the character's age." The aging is, actually, in days (around 50 to 150 days,) but we're not tracking age on the day-to-day level; instead, we give a chance that the character has aged enough to be visibly one year older. Spells that heal 2d6 damage can potentially age a character 2 years: one year for each die result of 5+. This eliminates the need for extra rolls, too.
For magic in general, maybe casting spells under certain circumstances (without prep?) requires rolling 1d6 per spell level, with each 5+ aging the character 1 year. There's actually a certain precedent for this (Sinbad and the Eye of the Tiger, for example.)
Brendan mentions another problem: tracking both effective age and actual age, two numbers instead of one. At first, I thought I would have an even worse problem, since I want to use age as, effectively, skill points for my background system, and we get added complications of looking up age categories for various races, if we're using something similar to the AD&D aging tables. However, *actual* age should really only come up in the extremely rare circumstance where you have to answer the question "was the character alive at a certain time?" Such as when a slow-aging character returns to a town where he might be recognized, or when a player asks "do I remember any details about the Third Imperial War?" It might be better to just record a static birth year somewhere and not change it at all; thus, you only have to track one number, effective age.
Thinking about this reminds me I should update my old aging rules, to eliminate the need for complicated AD&D-ish aging tables. I think my old system is still too clunky.