Professional sports are a young man's profession, and boxing more so than most. By their early 30s, the majority of fighters have already lost too much hand speed, too much off from their reflexes, too much bounce in their legs.
Modern sports medicine has given professional athletes a lot of tools, both legal and illicit, to prolong their careers. But nothing will ever change the fact that boxers, as a group, have a short shelf life. If a few rare fighters manage to continue to excel into their mid and late 30s, even fewer continue to remain competitive north of 40.
Some of this is the nature of the sport. If a former baseball superstar wants to hang around and be a .270 pinch-hitter/DH, it might be a sorry sight for his fans, but he isn't risking his health.
If the 43-year-old baseball player can no longer catch up to certain pitches he used to pull into the right field bleachers, it means he takes a seat on the bench.
If a 43-year-old boxer can no longer avoid certain punches, he is going to absorb more head trauma, after a two-decade career already spent taking head trauma.
Only a select list of fighters has remained competitive at a high level into their fifth decade on the planet.