Listening to the commentary of one of the Charles/Walcott fights and it mentioned that Walcott throughout his career fought in every weight class. Now I know he was 12 when he had his first amateur fight and 16 when he had his first pro fight. Correct me if I'm wrong, but I'm guessing that comment also referred to his amateur years.
Oh, including amateurs? Sure. I thought you meant pros and was going to say "nobody has, and can't imagine anybody ever will".
And Alabama Kid (112 - 188) Possibly also Willie Meehan... who they say began as a flyweight (though there are no weights available on BoxRec from the first half of his career), only to eat his way up to just under 200 lbs.