The one character I've had "retire" settled down to his keep and lands and let his son continue adventuring. He came out of retirement every now and then for more important stuff like invading armies from other countries and liches impersonating the good king.
Editted to add:
Nope, I've had one other adventurer retire after long campaign. I played him on in a private session with the DM after the campaign was over. He was a 17th level thief (several levels from end-of-campaign AD&D encounter where we killed Iuz) and switched to magic-user, invested his money in shipping, got married and had kids. Investments did alright but he had to brew potions to make ends meet. Eventually the DM had me roll my life span which led to an emergency and I had to go back to another old PC, the party magic-user, for help on my deathbed. Tired with married life I drank a couple of potions of longevity, grabbed my adventuring equipment and prepared to fake my death. We polymorphed a sheep into my character and tried to kill it. It made really good rolls and when even pushing it from the top of the wizard tower didn't do it in the magic-user just Power Word Killed it. After I took off, another PC who wasn't in on the secret reincarnated it and brought the sheep back as an elf. It was sort of a dumb elf and the wizard just shrugged and told the reincarnating PC "sometimes these things fail".