You get credit, but it's apportioned according to the achievement. Beating Ali is an achievement at any time. Frazier gets more for beating 29 yr old Ali than Berbick does at 39.
You're clearly a keen observer of the sport with a superbly developed sense of its history and legacy.
Big George. Duran. Malinga. Holyfield. Sanders 37 when he ko’d wlad. Hopkins. Azumah. Pascal. Froch. Glen johnson. Mayweather. Pacquiao. won world titles at 36 years +
Depends on the boxer. Those who rely more on physical gifts will inevitably decline more noticeably and often more rapidly than those who can adapt and compensate. Another important factor is how well they can look after themselves between camps.