Thank you, MeCorva.
It looks like there are at least two things to figure out:
- Specific age categories and associated modifiers.
- How to rejuvenate aged characters in a way that would be both interesting and available without making magical aging irrelevant.
I consulted the D&D 3rd Edition and d20 Modern rules, which use the following age categories (related ability scores changes in parentheses):
- Child (-3 Str and Con, -1 Dex, Int, Wis, and Cha; reduced speed [and size?])
- Adult (no modifiers, standard character - 12-year old humans are considered "young adults" already)
- Middle Age (-1 Str, Con, and Dex, +1 Int, Wis, and Cha)
- Old (-3 Str, Con, and Dex, +2 Int, Wis, and Cha)
- Venerable (-6 Str, Con, and Dex, +3 Int, Wis, and Cha [and reduced speed?])
Note that for each age category I listed total modifiers, and not cumulative effects as described in d20 System. No ability can be decreased below 3 or increased above 20, at least not for standard characters, and mental abilities (Int, Wis, Cha) remain unaffected by magical aging.
That's one way to put it. It's universal (same abilities are used in all editions of the game, even if they are listed in different order), descriptive (attractive to simulationists), and quite simple. It also has the added benefit of making game playable with younger or older characters.
But it can be simpler. What if each age category simply resulted in a bonus or a penalty to the skill checks? -1, -2 or +1. Or it can be something entirely different - let's discuss an alternative system for a moment.
The_Jester suggested making "aged" a condition. We have to remember that conditions can't be stacked, so there should be just two, without any steps: aged and rejuvenated. In this variant normal aging process is purely aesthetic and doesn't affect the character's mechanic. On the other hand, a character's age can be magically changed, which generally inflicts some penalties. Those two conditions cannot be applied to a single character at the same time. Rejuvenating an aged character (or vice versa) would return him or her to the standard age.
It's a truly abstract solution, which main advantage - the simplicity - is also its biggest disadvantage. It either won't address the concept of an elder character seeking eternal youth, and can't be successfully used with game effects like the Potion of Longevity, or it'll make rejuvenation something mostly or solely beneficial. I think I'd rather go with the first option and use age categories with modified ability scores instead of simplified conditions.
As for the second problem, I see it as something mostly campaign (DM)-dependant. Potion of Longevity is described as "rare" in the playtest packet, and it's the only item in the entire game which can rejuvenate a character. Besides, drinking it always involves a risk of reversed effect. So far nothing suggests that even high-level characters will be able to easily buy and hoard any magic items, and especially not those in "rare" and higher categories. The only way to unerringly obtain a single vial of the elixir is to cast a Wish spell, which requires an 17+ level character or a literal deal with the devil. If my suggestion to make aging a side-effect of the Wish spell on the caster is followed, them it'll be a suboptimal solution at best, as it'll be a chance of rejuvenation for someone at the price of one own's life.
Other game effects can be introduced as well - for example, a high-level (7+) necromancy spell which would age the target while rejuvenating the caster (or another character?) on a failed saving throw. In Pathfinder RPG, there is an Age Resistance spell, which doesn't rejuvenate the character, but lets him or her ignore the detrimental physical effects of aging (natural or magical) for one day. To make it something more common, Restoration clerical spell can affect the magical aging, but at least for me it's a bit too much. As unnatural aging is supposed to be optional and uncommon anyway, I think it should stay connected with the Potion of Longevity. If it seems too much, maybe it could become a "legendary" magical item, and find a "rare" replacement in an Elixir of Youth, which effects would wear off over time.