He would have certainly been in the mix and more than a four round fighter. A 6-6 southpaw with good boxing fundamentals, sharp countering abilities and top level power? That's a combination that will never go out of style.
Definitely an interesting what if. He has some really impressive attributes. His power was up there with Wilder, and he had some god given reflexes. I think that both he and Joe Joyce would have been champions, had they got into the professional game earlier.
A lot of his conditioning issues had little to do with age. His best wins were also over aging fighters.
Generally the younger the better so yes. It might have made a difference for the better. In addition to having a ten year earlier start, there are some things that would benefit him to keep and other things he’d have to do differently such as better conditioning . Much of it would have had to do with who he had as a trainer and a manager.
What if he put more effort into fitness and came in at a trim 250 lbs and not a low stamina sluggish 290+?