These threads are always started by stupid people.
There are no rules about when a fighter is or is not in his prime. Some fighters peak early, others peak late.
There are many factors that determine this, not just fight mileage (of which Fedor had much more, considering his Sambo background), but also genetics, training methods, motivation, fight style, punch resistance, etc…
Fighters like Ninja Rua might flame out very quickly, while others like say a Bernard Hopkins peak late and have extremely long careers.
Fedor fans were already talking about him being past his prime when he was beating guys like Brett Rogers and Andrei Arlovski. He had clearly devolved into a headhunter by that point.
To pretend that Brett Rogers and Arlovski were somehow more legitimate threats to Fedor than Big Nog or Cro-Cop is retarded.
Fedor didn’t start losing to B-level fighters because he got exposed in America. He started losing because he wasn’t remotely close to being the same fighter he was in Pride.