18. Roy Jones Jr.
The only boxer in history to start off as a welterweight and win a heavyweight championship, Jones was phenomenal in his heyday. He used his superb athleticism, movement, and dynamism to great effect against the likes of Bernard Hopkins and James Toney. Jones had incredible hand speed and was just too sharp for all of his opponents. That was until the years began to wear him down.
Jones lost just one fight in his first 50. But after this, the losses came thick and fast. Soon he was fighting for WBU titles and people with names nobody ever heard of. By the time he retired he was 49. Overall, it was about 14 years too late. The Russian citizen continued to risk his health for very little reward. We’re not counting his recent exhibition against Mike Tyson here because that was just for fun.