Lewis was the same "age" as Tyson and Holyfield when he fought them, but Lewis had hit the zenith of his career, while the other two were on the way down. Some guys hit their stride later than others.
Tyson's style and build, for example, probably would have meant that even if he hadn't gone to prison, he would have started to slide downwards by the time he was 28-30 years old. While the skill and speed were still visible in training sessions, continuous fighting of that style would have taken a toll on his body, much as it had Frazier, Patterson, and Marciano. Tyson had a second layoff prior to his fight with Lewis, giving him a total of 5 1/2 years of on-and-off inactivity during Lewis' uprising.
Holyfield was completely drained after fighting a rediculous series of wars. The man simply needed a break and just got overmatched by Lewis (though the judges "gifted" him differently.) Lennox Lewis had not gone through this kind of strain.
Lewis' body was large, and fairly mobile, giving him the kind of longevity that the infighters just couldn't possess. Remember the early stages of Lewis' career, however, where he was a lanky, awkward guy with a wide stance and terrible defense. He won against a lot of nobodies before his skills developed, and he really didn't prime until he was almost 30 years old. Because of this, I have trouble believing that he was truly the victor over Holyfield and Tyson. Nonetheless, he was definitely a great champion, and I am one of the few who even place him in the top ten. Much of the 90's hinges on the fact that Tyson was absent for so much of it. Had Tyson never gone to prison, the 90's would have perhaps been the strongest decade for heavyweights. Lewis, Holyfield, and Tyson, among many others (ie: Bowe, Foreman, Morrison, Mercer, Moorer, Rahman later on, etc.), battling back and forth for titles. This is one of the reasons why I rate him above Lewis; his absence almost single handedly brought down the division.
As a side note: I never thought that Rahman was terrible. When he was in shape coming up, he was really a solid boxer. I remember his first fight with Tua, when both of them were in good shape. Even though Tua scored an extremely late (and dramatic) stoppage, Rahman was punishing him all around the ring for 10 1/2 rounds. He was cut, with solid defense, and had a good jab.
In a randomized bracket:
2. Muhammad Ali vs. 10. Lennox Lewis
5. George Foreman vs. 8. Sonny Liston
3. Rocky Marciano vs. 7. Mike Tyson
6. Joe Frazier vs. 9. Jack Dempsey
4. Larry Holmes vs. 1. Joe Louis
Who wins? Who faces off afterwards? What's the final outcome?