The answer is Iron Mike Tyson. More notably than being the youngest to knock out Trevor Berbick, Mike Tyson is the youngest Heavyweight Champion of ALL TIME. Not an easy record to beat as Iron Mike surpassed Muhammad Alis record to win his own. Alis record seemed unbeatable for a long long time, but to surpass the new youngest age ever set by Tyson could very well take a century or two. The vast majority of Heavyweights do not fully mature until their mid to late 20s.