Previously I brought up the idea of aging as a potential side effect of healing magic. My original method was cumbersome (1 day of aging per HP healed) and potentially not salient enough to the player. Talysman also weighed in, and suggested that there could be a chance of years worth of aging per die of healing rather than a smaller guaranteed amount of aging. Talysman’s method avoids the need for extra rolls, but the probabilities result in aging being a bit too frequent for me.
I do think the idea of aging one year every once in a while rather than aging days for every instance of healing would work much better in terms of bookkeeping, and also be more salient to the player. I personally don’t mind an extra roll, since healing is infrequent. Based on these principles, here is a revised proposal:
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.
The percentage can obviously be tuned for any specific campaign. For example, if you want the aging to be rarer, you could make it % equal to half the HP healed, or a flat 1 in 20 chance (essentially, a fumbled constitution check). I kind of like having the number of HP healed and the character’s constitution influence the result, however, and HP healed = percentage chance of aging is really easy to remember. Also, if natural healing is 1 HP per day, that means that the recover of 350+ HP is the natural “worth” of one year of aging. Healing the same number of HP magically over the same time period would result in about 3.5 times more aging, making magical healing not a good alternative for the common cases.