We do not force players to retire characters, even when they pass what would be considered the threshold of "old age". However, we do encourage players to RP age appropriately!
No one is going to say "Okay, your character just turned 70. You can't bring him on quests anymore, and you shouldn't play him anymore." However, we are an RP server, and your character is aging. While ultimately you get to decide at what age your character is probably ready for retirement, it is always good to see players treating age with real consideration.
And personally, I think there's something to be said for letting characters finish their eventual story arcs and fade in glory in the sunset ... but then, I can't really talk, because let's face it, I play elves who live for centuries.