It's been said, but can never be said enough considering you seem to genuinely believe the stuff you type - How many of those guys you have listed even played a slam final at their favourite slam at the age Federer was in 2014? Let alone 2015, and 20 friggin' 19.
Djokovic in Australia this year was pretty much the same age as Federer in 2014 Wimbledon if my math is right. Month or two younger maybe. AND HE GOT TAKEN TO 5 SETS BY DOMINIC THIEM! Now yes, he won and Federer lost, but then you ask the question of who is better, 26 year old Thiem or 27 year old Djokovic.
Nadal in Paris last year was the same age. He came through in 4, but that was certainly the toughest test he's ever had in a final there, and had he not had the extra days rest who is to say how things might have gone.
Agassi won in Australia in 2003 at a similar age in what is widely heralded as one of the clown draws of all time. Had Federer gotten to face Schuttler I think he might have been alright.