It's a bit of a shower thought, but:
Instead of age bands and multiple modifiers (as in D&D, for example), why not just rule that when a player character is significantly 'older' or 'elderly':
- they must rest twice as often and/or twice as long
- rates of natural healing are halved
- optional: saves vs. disease at disadvantage/penalty
This hasn't been tested, but in principle I prefer it to unsoakable permanent Ability Score damage and penalising attack rolls.
Some of this came from mechanics; some from surveying my own decrepit corpus.