First of all, older superstars do take the spotlight away from younger stars, just because they have so much name recognition that giving them titles will certainly bring in business. For example, when looking at WWE's mid-carders, you see a lot of talent that is ready for World Championships, and at the same time there are by my count 11 active wrestlers in the WWE who are former World Champions, 4 of which are over the age of 40, 2 in their late 30's. I'm not saying these people need to retire or can't pull their weight in a match, I'm just saying, it may be better for some real good talent if they were to retire.
Second, as good as a wrestler may be, if they are spending the majority of the year sidelined because of injuries, I think that is their body's way of saying it's time to retire. It's tiring to see these wrestlers constantly being written off television for injuries, only to have them come back and take the title again, only to drop it when they become injured again.
Third, Ric Flair definitely needed to retire / still does? (I don't watch TNA so I don't know if he's still having matches). He may be viewed as the greatest wrestler of all time by some people, but when I got turned back onto wrestling about August 2007, I would see Ric Flair come out and just ask myself "How am I supposed to believe that this man is supposed to be a legitimate threat to anybody on the WWE roster?" But he just kept wrestling until he got "retired" at WrestleMania 24 at the age of 59. Funny enough, I saw Scott Steiner's infamous promo saying Flair was too old to wrestle, and that happened when Flair was 51.
So pretty much, the official age to stop wrestling at is when people no longer believe you should be able to legitimately beat other wrestlers.