Well i know Mike Weaver was about 2 yrs younger than Holmes so i will nominate him, though no doubt someone will research and find definitive answer.
Wikipedia needs to do an update. Check out the link below. http://www.boxingoverbroadway.com/doug-jones-boxer-gave-young-cassius-clay-toughest-fight-dies-80/
Although none of these ever won the title, they all challenged at least once for the heavyweight title and are still with us as of this posting: Pete Rademacher at 89 Brian London at 83 Karl Mildenberger at 80
Geeze, German heavyweights seem to be made of pretty stern, or at least healthy, stuff. Look at Schmeling.
Very sorry to learn this, he was terrific competitor against some of the best heavy and lightheavy boxers of the 60's. His image came up in my mind a few times this week when news that his namesake had won an election for the Democrats.