If you get KO'd you should retire. Only in Boxing you hear this. It's because people are quick to jump on the wagon and secretly love it when a fighter they dislike gets KO'd. Khan should not retire. No chance. 25 years of age I think. He's no where near done.
In boxing is different than other combat sports in that boxers who are great prospects really can't lose because of the paydays involved. You want to see two great fighters in their prime, undefeated face each other, it's part of what makes boxing great. I don't think Khan is not done but I think he has some serious *****s in his armor and people tend to see that. So when I think they say "he's done" I think they are referring to pound4pound greatness.