The idea of retired characters came up in the comments on another post the other day. Basically, I dismissed the idea that "Stericksburg" would be renamed after an active PC because he was pretty badass. Maybe when he's retired, I said.
When would that be?
No Mandatory Retirement Threshold
One thing about my game is that it's all about hack-and-slash fun.
So as long as someone is having fun playing their hack-and-slash guy, they can keep playing that guy.
There does reach a power level when you're basically not challenged by much of what you encounter. But GURPS is GURPS - there is never a level where you're effectively invulnerable. And no matter how powerful you get, there are things you just might not be able to deal with.
A perfectly lethal knight won't ever be able to overcome some magical effects, especially ones that automatically overcome resistance and need post-effect remedy, not a good roll.
An epic-ly powerful wizard won't be able to overcome foes in melee in a NMZ.
A powerful cleric might wave away undead with trivial ease but lack to the ability to destroy non-undead with sufficient ease.
So I'm not especially concerned. As long as people are having fun with their guys, they can run their guys. I'll let them sort out the who and what and when of stopping playing a particular paper man. And if someone wants to basically set a PC aside for special occasions, that's fine too. We've got a dozen or so templates to choose from and lots of levels, side levels, attached side areas, and so on to explore in the megadungeon game. It's all about the player's fun, not the PC.
One of my players did say, basically, that he means to keep playing one of his guys until he dies and can't get brought back. That's not counter to the game. You don't win in DF Felltower by getting to retirement age or being unbeatable. You win by risking your paper man to have fun. And as long as I can keep up the challenges, why set a PC aside?