Nole beat peak Fed in AO 2008 in 3 if i recall But ofc Olderertads will say he was past his peak at 27 Funny when he started losing from better players and getting owned right and left his 'peak' suddenly went away.
Firstly, I don't think it's that unusual for a tennis player to be past their peak at 27. Not everyone is Ferrer.
Secondly, if you look at Federer's results in 2008 onwards (which I'm sure you did before posting) you will see he started losing more than he had done before, and to some players he would expect to beat, as well as your favourite Djokovic.
Thirdly, I think that when you start losing more than you did before, and can't reach the same level of play on a consistent basis, then, you know, you're probably past your peak...