Don Meredith and Don Perkins left the Cowboys at ages 31. Four-time Pro Bowl offensive tackle Pat Donovan was 30 when he turned in his pads. Jim Brown quit the Cleveland Browns as the NFL's all-time leading rusher at 30. I recall John Riggins retired for all time and didn't return to the Washington Redskins for an entire year. Retirement motives vary.
Athletes use the threat to re-negotiate a contract.
Others to force a trade. I suspect a few use retirement to raise wails of distress that feed the ego. A few like Buffalo Sabres goalie Dominik Hasek retire a year in advance, raising the prospect of a mawkish Farewell Tour. There are also cases where athletes quit to pursue odd ambitions. Too Tall Jones boxed for one year before returning to the Cowboys.