Bobby Dobbs fought over 5 Decades, a picture of him appeared in the RING Magazine, 1940s, with a mature older looking face and a physique like a Young Man.
It's Moore no contest. Fitz deserves a nod too.. won the LHW title at 40.. & 40 back then is older than what it is now.. Usyk & Pac are good modern examples too.
100 % a strong argument could be made for him being among the absolute TOP HW's and fighters of all-time, period. an absolute Great skilled, stylish, cagy & dangerous fighter for anyone.
Definitely Moore. Hopkins got some stuff done as a slightly older man than Moore, but he had nowhere near the culmination of wear and tear at the world level Moore had. Hopkins started late, to the point where he only had 2 ranked opponents on his ledger by the time he hit 30, and about 30 fights. At the same age Moore had already faced Ezzard Charles, Jimmy Bivins, Teddy Yarosz, and all members of The Murderer's Row several times, and had more than 100 fights total. If he was anybody else he'd be slipping soon. For him to become champion at 36 and still reign for 8 years while defending against actual top contenders, and to still be knocking out top 10 rated Heavyweights at 46 like Lavorante, historically insignificant as he might be, is nothing sort of unprecedented given what he'd done up until that point and who he had faced.
I would go with Moore as well, however, a few more could have been added if you wanted to make a top 10 (or more) list of some outstanding aged veterans of the game. Offhand, I could also think of Eddie Cotton, Jesse Burnett and Baby Vasquez.
While Malinga certainly wasn't the greatest 40+ boxer, he won two titles at 40 and 42, scoring two biggest wins of his career vs Nigel Benn and Robin Reid.
Saoul Mamby deserves a mention. He beat an undefeated Glenwood Brown at 41 and went on to fight sporadically until 60. Always a crafty operator with an iron chin.
For his part in the 8 knockdown thriller with a guy, Durelle, who was at least fifteen years younger than him, Archie deserves a top spot!!