Hopkins practised a professional life both inside and outside of the ring. In sharp contrast to some of the more popular fighters of modern times (Duran, Morales, Hatton,) he tends to his body as an implement that is essential for his work and as such must be cared for and serviced to ensure it's longevity.
He is a guy who's training is entirely technical (because he's always in shape he never needs to get fit) This maintenance is less hard on the body than short periods of high intensity training used by other boxers.

Supplementary he restricts his diet and is constantly mindful of the effects of his actions on his fighting. Finally the solid defence he has developed has prevented damage.
All in all it's down to sheer will and intelligence.