Why are modern heavyweights so old

One more thing: Cubans, Russians, and Eastern Europeans are in the mix now. These guys tend to fight long am careers and turn pro at a later age--not a rule, but a tendency.

 

Athletes keep their abilities to a higher age nowadays, not only in boxing. This is in part down to conditions in general being much better and life less hard (just look at your grandparents in their early 30's and compare them to yourself or other people in that age today) and also down to modern training being more effective. Advances in knowledge and science makes it easier to construct training programs that maximize effects and minmize wear and tear.

Of course, the fact that fighters in general fights much more rarely today also plays a part. But for example footballers play much more frequently and at a much higher pace, but still keep their qualities longer. This is, as I said, a general phenomen in sports, not only boxing.

 

Bingo.