15. Troy Aikman – Retired at 33 in 2000
Entering his mid-30’s as part of a Dallas Cowboys team that seemed to be further and further away from its glory years of the mid-1990’s, Aikman was still a very good quarterback (although not as good as he once was). But, he was starting to feel the effects of all the hits he took early in his career, missing seven games during the 1998 and 1999 seasons due to injury.
In the 2000 season, in what would be his final home game in Dallas, Aikman was scrambling out of bounds when he took a violent hit from Washington Redskins rookie linebacker LaVar Arrington. The impact of the hit caused him to suffer the 10th concussion of his career. Aikman never played another down in the NFL.