At 19, Nadal reached the finals of Wimbledon in 2006, losing in four sets, then got to the finals again in 2007, losing in five sets. The following year, they had the famous 9–7 in the fifth finals where Nadal finally broke through. While Nadal was still a teenager, he was 6–1 overall, and 2–1 on hard courts against Federer. He didn’t have the consistency on hard courts but still managed to win fours Masters 1000 hard court titles from 2005 to 2006, including his win over Federer in the Dubai final. In 2005, he blew a two set to none lead against Federer in the finals at Miami.
With regard to Djokoic, Federer is still winning roughly every other match. Do you not realize he beat Novak in the semi-finals of the 2012 Wimbledon? Federer was extremely unlucky to lose two straight US Open semi-finals in 2010 and 2011, as he had two match points in each match. So, your point about Federer being unable to win Slams after Djokovic made the leap is not supported by the facts.
With regard to Murray, Federer has been beating him like a drum AFTER he turned 30. He is 8–3 against Murray since 2012.
It’s even worse for Wawrinka. Federer is 13–2 since 2011.
The only evidence from Federer’s results are the slight drop in level due to age dealing with the grind of winning seven best of five matches in a Slam. Federer has proved that his effortless style and incredible training discipline allows him to play at the highest level, well past the age of thirty.
(Note: I totally disagree about Marat Safin. He had a small window from 2000 to 2005 where if he got hot, he could play better than anyone in the world. When I watched him beat Sampras in the 2000 US Open, I told my friends that tennis had been reinvented and you could throw out every book written about playing the percentages.)