It's well documented that Federer dominated every younger rival, except Djokovic. I contend that it's because Djokovic uses his superior return of serve!
Let's suppose when facing a Top 5 or 10 opponent, Federer wins 75% and 50% of his 1st and 2nd serves, conceiting 25% and 50% to his opponent. Well, when facing Federer, Djokovic just aims to win 28% and 53%. Those small margins will mean 2 more breaks over 5 sets, and the difference between winning and losing. Can Djokovic do that to Federer? Absolutely, because he's the acknowledged best returner in the game! That he's the best returner, and that he's the mentally strongest are enough for him to gain an upper hand on anybody (the one exception: Nadal on clay)! He doesn't need any age advantage!
How did the Federer-Nadal turn on its head in '15? What bedeviled Federer early on, was Nadal staying well back, and keep retrieving gorgeous shots from Federer! Well, Nadal predictably lost that kind of amazing speed, and now had to stand right behind the baseline! That was when they bought in Costa and Rogue to teach him the new game, and out went Tio Toni! When Federer and Nadal stand on the BL to duke it out, there can only be one winner. Because Federer has the more compact, efficient strokes; and he's much more used to taking balls off the bounce! Hence, 6-1 record, with that one loss coming on RG!