6. Pat Tillman
Retirement Age (Career): 25 (1998-2001)
Pat Tillman wasn't as celebrated on the field as the other stars on this list, but he's just as important to the culture. Pat was in the fourth year of a decent NFL career when the the 9/11 terrorist attacks occurred. Several months later, he sacrificed millions and enlisted in the U.S. Army. Unfortunately, he lost is life due to friendly fire on April 22, 2004 and his death is mired in controversy. Regardless of any political beliefs and conspiracy theories, there's no denying that Tillman was a true American hero.