It used to be normal for heavies to be washed up in their mid-30's. In my opinion, this all changed in the 1990's when Foreman made his comeback, Holyfield had a lengthy career, and Lewis was still winning in his late 30's. The Klitschkos are continuing the tradition. Prior to that, most heavies were done by 35....and that includes the great ones.
Let's look at the facts here... Riddick Bowe was slurring his speech by 1996 at age 28. He also lost most of his coordination, and muscle definition by 1996. In my opinion(I have no proof), Riddick Bowe had developed early stages of Brain Damage by 1996. Ezzard Charles had little to zero movement, speed, and reflexes left by 1955. His muscle definition dropped off dramatically between 1954 and 1955. Since he wasn't diagnosed with ALS until 1967, it's impossible for him to have picked up symptoms in 1955 of the disease. Therefore, I cannot explain such a physical drop off between 1954 and 1955. I also believe in 1951 at age 28(after the Joe Louis fight) he displayed his first drop off in physical ability(showed slight signs of speed, reflexes loss). Worse yet, from 1955 to 1958, his drop off in physical ability became so severe, he was reduced to losing to .500 clubfighters. In 1948, he was one of the 5 greatest fighters ever to grace the earth. 10 years later, he wasn't good enough to shine the shoes of a C level journeyman. Sad.
I have thought about what you said, and yes, you are right. Louis held the title for almost12 years, so he is not an example of a heavyweight who aged prematurely.