Ageing is something that seldom comes up in games, except in cases of artificial ageing by curses or something. Most campaigns don't tend to run for long in enough in game time for characters to age more than a few years.

What about running a campaign in which the individual adventures don't take place serially, in quick succession, but rather spaced out over years, with large gaps in between? You can think about TV series that were revived after a long time, like The X-Files, only take it as a model for multiple gaps between adventures.

A young group complete some adventure, and are so exhausted and perhaps wealthy afterwards that they retire. But a decade later the old threat resurfaces, and they are the best group to fight it again, so they regroup. Then several years later again, something happens which has weird echoes of the previous two adventures, and the heroes need to come together a third time, now much older (and hopefully wiser).

Transcript

GM: Make an opposed Strength roll, with your Force bonus.
Luke: 13.
GM: The helmet comes off, leaving just a facemask.
Darth Vader: Finally... release.
Luke: Mother, no...
Darth Vader: It is done, my son.
Luke: Can I... see your face? I never knew you as my mother.
{takes mask off}
Padmé: I haven't aged well, my son.