Some heavyweights seem to age prematurely. Joe Louis looked ancient in his mid-thirties. Jerry Quarry was washed-up at 28. Any others?
Quarry certainly aged badly,...he was thicker after he matured and less quick and precise than in the late 60's. He was a faster, cleverer counterpuncher then..before he was "shamed" out of that style post Ellis.
Jack Sharkey went downhill fast after winning the title. He lost the title in his first defense against Carnera and only had a handful of fights after that, losing most of them. Arguably he was already on the decline when he won the title in a very unpopular decision against Schmeling. Check out the Dempsey-Sharkey (1927) and the Louis-Sharkey (1936) fights on YouTube. Compare the physical appearance of Sharkey in each. Hard to believe it is the same man. Joe Louis, as mentioned above, seemed to age noticeably between the Mauriello and Walcott bouts in 1946 and '47, respectively. And he really looked much older when he came back against Ezzard Charles in 1950 when he weighed a career high of 218.
some guys get ruined from drugs and some from a bad loss, Quarry from drugs and booze, Dokes from Coke, Willie Dewitt from mismanagement and poor training, Nick Wells from lack of management, Jimmy Young fell off track and lost motorization and Joe Frazier burnt out from partying and sudden fame, Dempsey from inactivity
Louis is a strange example to use considering he is the longest reigning HW champion in history. He lasted a very long time and didn't waste his potential.
Louis was world ranked until his final fight. He may of lost a step by then, but he had not aged 'prematurely'. Indeed he is an example of fighter defying time.
Louis only looked old after he retired and became a drug addict. After he KO'd Walcott in his final title defence at age 35, I remember watching the post fight interview where he looked quite youthful of body and didn't have even a wrinkle on his face. Hairline was receded a bit but that was it.