Steve Young – Retired at 37 in 1999
Steve Young might have been approaching the twilight of his career, but he was still one of the NFL’s elite quarterbacks. Young was named to the Pro Bowl every season between 1992 and 1998. In 1998, amidst questions as to whether his skills were diminishing, Young had arguably the best season of his career. He threw for career highs in passing yards (4,170) and passing touchdowns (36), and took the 49ers to an NFC Divisional Playoff appearance (defeating Brett Favre and the Green Bay Packers just one week earlier — which included the famous last-second touchdown pass to a young Terrell Owens).
But early in the 1999 season, Young was knocked out of the game after getting hit by defensive back Aeneas Williams of the Arizona Cardinals. That was already his second concussion of the season — and at least the fifth of his career. The repercussions of Young continuing to play after that were so dangerous, the 49ers threatened to release Young if he attempted to get back on the football field. He never played a game in the NFL after that.