"Old guy" is a relative phrase. Anyone in his 30s is usually considered in the prime of his life; however, in pro football, especially at the quarterback position, a player is usually beyond his glory days by 35.
Players over 35 risk more than their reputations by clinging to one more shot on the football field. Most risk serious injuries, ones that will—at the very least—create lifelong crippling pain and—at worst—shorten their lives. Football is a young man's game. Players are bigger, stronger and more intimidating than when many of these over 35-year-olds started their careers. Hits (even those not incurred under the bounty system) can be devastating. How many neck surgeries does one man need before he realizes that he is risking his life?
Granted, Johnny Unitas and George Blanda succeeded beyond that mid-30 mark, but they played in a different era and Blanda was smart enough to switch to kicking in his twilight years in the NFL.
Chad Ochocinco, Randy Moss, Hines Ward, Aaron Smith, Donovan McNabb and Terrell Owens, to name but a few, have all reached the point in their careers where they should question the quality of their contribution to a team. To a man, they have had successful runs in a league that quickly "chews up and spits out" only the best. Why not retire when stand-out performances resonate in the fans' collective memories? Brett Favre would have been remembered with reverence if he had hung up his helmet in Green Bay. Now many fans speak derisively of his attempts to revive his football career in New York and Minnesota.
Peyton Manning has proved himself. Money cannot be a factor. With a constant stream of endorsements, not to mention his Colts salary, he is likely wealthier than most hard-working stiffs. He won one Super Bowl, but surely he's not hanging around because younger brother, Eli, is now sporting two Super Bowl rings. Peyton is now a parent with young twins—they deserve to grow up with a father who recognizes that glory is fleeting but without health and well-being, it is hollow indeed.