He's 31 years old, that's all the proof you should need. Name me one modern player who was as good at 31 as he was at 25-27. Agassi was good in his old age, but nowhere nearly as good as he was in his prime. 25-27 are prime years in tennis. After that comes the inevitable decline for ANY player. What Federer is doing at his age is nothing short of amazing. He regained no.1 spot for a while and won Wimbledon last year. At his age, that is a massive achievement. Granted, the difference between this Federer and 2006 Federer may not be huge, but in tennis losing half a step and/or a little bit of stamina is the difference between a grand slam champion and a runner up/semi-finalist.
I'm Nole fan, but in 4 years from now, when Nole starts losing frequently, are you going to make the same "prove it" argument? I am almost 100% certain that at 31 years of age, Nole will not be nearly as successful as Federer is right now.
For a while, I was of the same opinion, but then I re-watched some of Federer's matches from 2006/2007. There is no doubt in my mind that this version of Federer is (relatively speaking) a shadow of prime Federer. You may attribute that to the rise of competition, and partly that is true, but competition or no competition, it is clear that at his age, Federer is just that much slower and not as fit physically as he was back then, and that's all it takes.
Decent, objective, and most of all, SENSIBLE Djokovic-fans. Oops. I should say TENNIS fans. So they do exist.