Saying his illness is a result of carrying on too too long is a massive oversimplification, if not completely incorrect. Fact is, he had his last fight before the age of 40 - not exactly 'old' for a heavyweight - and only 4 fights beyond the age of 35.
The problem with Ali, is that he fought arguably the 3 most devastating punchers of all time in Shavers, Foreman and Frazier - and took a lot of punches in those fights, especially to the side of the head. His tactics against Foreman probably did the most damage.