It's not just the heavyweights. I remember when Duran and Arguello were considered old by boxing standards at the age of 30 or so. Today we don't think twice about a 32 year old fighting big fights. But back in the 70's and 80's, the general feeling was that a guy that age had a lot of wear and tear and would be disadvantaged against an active 22 year old. Times have changed.
For the record, here are the ages at which all current outdoor athletics world records were set. I suppose you might argue, with the strength element, that the throwing events might be somewhat similar to heavyweight boxing, at least in that people with not dissimlar body types might get involved? https://www.worldathletics.org/records/by-category/world-records 100 Metres: 23 200 Metres: 23 400 Metres: 24 800 Metres: 24 1000 Metres: 21 1500 Metres: 24 One Mile: 25 2000 Metres: 25 3000 Metres: 20 5000 Metres: 24 5 Kilometres: 23 10,000 Metres: 24 10 Kilometres: 20 20,000 Metres: 34 One Hour: 37 Half Marathon: 27 Half Marathon: 24 25,000 Metres: 26 30,000 Metres: 26 Marathon: 34 100 Kilometres: 35 3000 Metres Steeplechase: 22 110 Metres Hurdles: 27 400 Metres Hurdles: 26 High Jump: 26 Pole Vault: 20 Long Jump: 28 Triple Jump: 29 Shot Put: 24 Discus Throw: 26 Hammer Throw: 31 Javelin Throw: 30 Decathlon: 27 20,000 Metres Race Walk: 24 20 Kilometres Race Walk: 27 30,000 Metres Race Walk: 36 50,000 Metres Race Walk: 33 50 Kilometres Race Walk: 37
Heavyweight fights today are fought at a snails pace, and for fewer rounds. The big heavyweights have copied the Wlad method of fighting. Control distance and keep the opponent on the end of your punches, Wlad always lost if a fight broke out like Joshua did when he had to fight Ruiz. I don't think any of todays heavys could get into a fast paced fight and last even 12 rounds much less 15.
The Joe Joyce - Daniel Dubois fight was pretty interesting in terms of styles and the ages of the fighters [35 and 23 respectively]. As per your comment about pace, Joyce's style is, putting it very mildly, durable... he's kind of the anti-Tyson, the 'juggernaut' style and peek-a-boo are pretty much diametric opposites.
