Disagree. I'd say Calzaghe's peak was around the Mitchell fight. It was only due to Frank Warren that he didn't get the REALLY big names
There are always many factors in a thing like this. Styles, and how much you are relying on qualities that naturally fade. How much you got hit and how many wars you had. Dedication to training. Dedication outside training - eating healthily and living correctly. How smart you are to adapt. It's also worth remembering Calzaghe's old school training. He is 36 now, throws 1000 punches with absolute ease, and is still relatively quick. He never pumped weights or anything like that. The old school fighters also fought 15-20 rounds regularly. There isn't a coincidence in this.