By active I don't mean current fighters, but fighters who fought a lot, frequently and with a long record when they retired. Who are some of the best? Robinson, Greb and Armstrong come to mind
Jack Britton. In 6 years from his debut he fought near a hundred bouts! He fought for 25 years overall, and retired at 44, fighting top competition for all of those years. In total, he had 344 recorded bouts, and fought a who's who of ATG's, beating quite a lot of them, too. A true Boxing marvel.
Pep averaged a fight every 6 weeks for 26 years. Throw in a plane crash and you've got one tough active hombre … Pac takes it for me with strength of resume and activity with the moderns
Archie Moore. Didn't slow down until late in his career. Ezzard Charles. Also, don't forget, he lost several years in his mid-20s because of the war.