I can understand, why one would think of Carpentier, who was only 17 when he had his first EBU title fight... but he was no more than than 28 for his last - so he's not our man.
Yes, Sir... van Klaveren is the man, we're looking for:good He was 2 months shy of his 24th birthday, when he had his first EBU title fight in 1931... and 2 months past his 48th birthday for his last in 1955!