I think that age has to do a huge invisible part, do you think that more than 5% of the baseball players hit 50 homeruns at age 38 years old and bat 324? no!, they probably avg 252 and hit like 15 if they are extremely good (and they don't even get punched in the face for decades)

in the current age of advanced nutrition and training that has produced so many old good boxers, but anyways, not everybody possess all the longevity that others do

now for practically all but not all boxers, it affects a lot more that what people see, in not
just 1 aspect like timing but a lot more ones, as many boxers are similar to many others in level, the difference is so little that if there is a little bad training or slipup or whatever and you lose the match

And about the match between lewis and klitschko, it was very even but I think that Klitschko would have won, although maybe not in their primes, both are very good boxers for my tastes of not being addicted to 50 raw hard punches with no defence per round with shoes cemented to the floor