I'm not a Fedor fanboy, but your argument crumbles when you say things like, "33 is not old for a HW MMA fighter."
First, in just about every sport, being 33 means you are on the backside of your career. A man's athleticism starts to wane well before reaching the age of 33. That fact isn't even up for debate.
Second, every athlete ages differently, which is dependent on a number of factors (e.g., genetics, quality of training, nagging injuries, mileage on the body, etc.).
Third, Fedor's style was never conducive to longevity. He was a small HW who depended on being a fast twitch athlete (i.e., explosion, quickness, reflexes, etc.). Those are the first attributes to decline as a man ages. Whereas guys who rely on being physically power or who are grinders are able to stay relevant much older. There's a reason why guys like George Foreman and Bernard Hopkins were champions well into their 40s (albeit not as good as when they were younger), while someone like Ali (and other fast-twitch boxers) was completely shot in his mid 30s.
Fourth, MMA has done an awful job of developing young HW prospects over the years. Hence, there has been an abundance of old veterans who have remained relevant. This isn't indicative of the fact that "33 is not old for a HW MMA fighter." Rather, it's due to the fact that not enough young lions have come along to replace the old ones. Whereas most divisions have gotten better and deeper over the years, HW has grown thinner.