Ray Mercer started boxing at 23

Yeah, perhaps but only because we would have all been ignorant of the fact that Mercer can't even beat a 42 year old Holmes.
As it stands we could bring ourselves to admire Holmes for beating the guy, and then Holyfield for beating Holmes.
Seems just as fair, doesn't it.

-- Holyfield always said he didn't really want to face Holmes due to the 'no-win situation' of fighting another old man but promised the winner of Mercer-Holmes (everyone expected it to be Ray) a shot at the title.

 

Foreman did something similar. Started at boxing in 1966 (age 17) and and won the Olympic gold in 1968 (at 19), he had 26 amateur fights in total.

I think 17 and 18 is stil in the ordinary age range to start boxing. 23 is really significantly older.

 

Ray, in shape could be as tough as they come. But he should have never lost to Holmes (arguable - Larry fought a great fight) and Ferguson.