I was watching some Leo Santa Cruz the last two days and it strikes me that he entered his prime in 2015 and arguably hasn't exited it yet. That's already long. What are some of the longest primes? Extra points if you can find guys who had long primes that include losses. Edit - Primes, not longevity, not doing well post and pre-prime. Maybe state when the primes you are claiming ran too and from.
Hagler is the name that immediately springs to my mind. He may well have entered his prime by the end of '77 (Colbert); peaking in the early '80s (hard to tell exactly when, but Sibson seems a suitable high-point, in '83) - continued to look pretty good, as far as early '85 (Hearns) - only to grow old fast, between Hearns and Mugabi. For over seven years, he looked in mint condition. No losses during that period, though. Only a disputed Draw (Antuofermo).
Duran and Hearns. Duran in particular. He turned pro in '68 and of course captured multiple world titles in different divisions. He was a dominant fighter in the 70s and 80s, and even made a good run in the 90s, defeating Jorge Castro and giving guys like Vinny Pazienza a tough go as well.
Mayweather and Hopkins certainly. Every time they lost a step physically they improved a step technique wise.
I agree. Both actually got better the older they got. BHOP was a rough and physical middleweight at first that developed into a brilliant technical fighter. Vlad calmed down as he got older and fought with much more intelligence and better defense as he aged.