I think they should retire when there performance starts slipping. It seems kind of obvious when stuff like that starts happening. And it seems like many people look at retiring like its a bad thing, I think its extremely admirable for the older guys to hand down the torch to the younger generation. Like everyone else said, Couture went out at a perfect time. He knew he was not a title contender anymore. Many of these guys may lose a fight but are still in title contention because they CAN beat the champion, but couture knew he couldnt beat Liddell again. And I'm not saying this to disrespect Couture, he is a legend and always will be but he understood the game. Whereas Shamrock was still fighting for no apparent reason (other than revenge). So basically when you don't think you can be a contender then you should retire.