Heroes do get old. Some characters are also relatively aged when their adventurer career starts (for example, wizards). Here are a few optional rules for age to affect characters:
Beginning at 40 and then every 5 years, the player must succeed a Physical throw for his character. Should it fail, the Physical score is reduced by 1. All the scores depending on Physical are recomputed (Strain, Strength, Coordination, Speed, and the skills depending on them.
Beginning at 50 and then every 5 years, the player must succeed a Mental throw. Should it fail, the character's Mental score is decreased by 1. All the scores depending on Mental are recomputed (Strain, Will, Decision, Intelligences, and the skills depending on them.
When Physical or Mental reaches 0, the character dies, after a well-lived life.
Légendes la Table Ronde, core rules, 1986
(Strain points equivalent to D&D hit points are computed as Physical * 2 + Mental)
This comes from First Legends, a game without character levels. Experience leads to skill score increase, and ageing leads to attribute score decrease.
A saving throw against ageing is easily portable to other systems.
You do not become invicible, you become weaker. Your power grows, but you're not holding the dagger. Your head cools, you're leaving the youthly gardens of the short term. The game has changed.
I would wonder if the decline of ALL the abilities would be a bummer for players and a bit nihilistic. However, I do get the physical parts...
I wonder about maybe taking the mental parts where a failed save may subtract that attribute, but maybe a successful save could increase it?
This is me thinking about the 1e/2e rules on aging where Int and Wis usually increased with age.
Should WIS augment from 10 to 40? If after forty, the setting doesn't change much (especially if low fantasy), there is not much simulation for the the characters to adapt, they're set in their habits.