Longest careers ever in boxing?

What medical explanation?

 

Him having ulcers and chronic pain in his prime. This being taken away gave Moore a boost at a time fighters are typically slowing down. Taking away a disability should counter the effects of aging because you're adding more then the aging is subtracting(at least for awhile).

 

Bob Fitzsimmons had a very long career in years 31.

 

And if you factor in his adult Amateur Career, which began in 1880, Fitz had a career lasting 34 years.