Master, Iaminut & Spiccoli best define it for me. We just vary on the age.

A man's prime IMO is age 25-35
There has to be room for rookie years (20-25).
By age 25, your mind, & body have peaked.

The next question is how long does it last.

Depends for boxers on 2 things.
1 Fight style
2. Life style.

Punchers & Brawlers Fraziers, Tysons , Tuas: bangers usually have shorter primes, they take more poundings.

Boxers- counterpunchers last longer because they take less punishment.

Some fighters also lessen their prime by doing too much sparring & training.

Wether it be basketball, football or baseball most athletes are done by 40. History bares witness.

Hop & PBF don't drink and party in that sense, so that lenghtens a prime to as long as 40. & Forman retired for years, giving his body rest, so he never really had a prime- He was fat when he came back & knew who to fight and not too. And since power is the last thing to leave a boxer, Forman was good to go at 40 plus. Good thing Holyfield wasn't a muderous puncher- he could've hurt George.