Gale Sayers and Jim Brown retired before their 30th birthdays. Barry Sanders, who retired days after his 31st birthday, was right behind them.
The premature ends to their NFL careers didn't stop any of the three from making ESPN.com's list of the top 10 running backs in history. Brown topped the list, Sanders was No. 2 and Sayers ranked fifth.
"You think about those three and wonder where all the records would be if they had played well into their 30s," said former Kansas City cornerback Emmitt Thomas, who will be inducted into the Pro Football Hall of Fame in August.
Sayers scored 22 touchdowns as a rookie for the Chicago Bears in 1965, but two major knee injuries cut his career short, and he retired in 1971.
"The sad thing is that if he were playing today and had those injuries, he'd probably still be playing,'' former Minnesota Vikings running back Robert Smith said. "There have been so many advances in the medical field that his injuries probably wouldn't have been a big deal. It's too bad, because he was an unbelievable runner. He had an innate sense, like he had eyes in the back of his head, and he always knew where the defenders were and how to avoid them."
The end of the road for Brown and Sanders wasn't as tragic, but it was just as abrupt.
At 29, Brown announced his retirement on July 14, 1966. He walked away as the NFL's all-time leading rusher with 12,312 yards, but many felt he was still in the prime of his career.
Brown's retirement came while he was part of the cast filming "The Dirty Dozen." Cleveland owner Art Modell insisted Brown leave the movie set and report to training camp. The running back instead chose to walk away from the game.
"He had other things he wanted to do with his life," said Thomas, who entered the league the summer Brown retired. "He's been very successful at a lot of other things, but you still wonder what he might have done if he'd played a few more years."
Sanders' retirement in the summer of 1999 was just as stunning. He was healthy, and his 15,269 rushing yards ranked behind only Walter Payton (16,726 yards) at the time. Sanders kept quiet about the reasons for his retirement at first but later admitted his decision was due to frustration about the continuing struggles of the Detroit Lions.
"Barry was such a special runner," said longtime NFL coach Dan Reeves. "You could play an eight-man front against him, and it didn't matter because he'd just bounce off people. He didn't always have a lot around him, but he always made it interesting. I think everybody wonders about what he could have done if he spent his career with a lot of talent around him."
Pat Yasinskas covers the NFL for ESPN.com.