I will now give what I consider as the priming point for Federer, Nadal and Djokovic, the signs and the reasons for picking each of these years:
For Federer it was 2004. His case is the easiest to explain - prior to 2004 Federer only reached 1 Grand Slam SF (at 2003 Wimbledon which he won), he didn't reach a single hard court Slam QF and only won 1 Big hard court title (the 2003 YEC which was just right at the end of his pre-prime). At the French Open, Federer reached 1 QF prior to 2003. Between 2004-2009, on the other hand, Federer reached SF or better in 23 of the 24 Slams he played, won multiple Slams in 5 of these 6 years and won 5 of the 6 hard court titles, 4 of them multiple times. The difference between Federer before 2004 and from 2004-2009 is as clear as it gets. The next time Federer lost before the QF stage of a hard court Slam was at the 2013 US Open, in a poor form year for Federer.
For Nadal it's not as easy to detect, but his priming point is 2008. The key to this definition is Nadal's results outside his favourite surface. Before 2008 Nadal didn't reach a hard court Slam SF, he didn't score a real top 10 win in any of the 3 non-clay Slams. He also had neither a grass court title or a real top 10 win on grass. Between 2008-2013, Nadal won 3 hard court Slams, reached the SF stage or better in 8 hard court Grand Slam editions (out of 10 participations), won 3 grass court titles (among them 2 Wimbledon titles) and scored 7 top 10 wins on grass. In 2008 alone he reached the SF stage of all 4 Slams. Such a drastic improvement on Nadal's non-favourite surfaces can't be overlooked.
For Djokovic it is not that difficult to detect, his priming point is 2011. Prior to 2011 Djokovic couldn't reach multiple Grand Slam finals in the same season, he didn't reach the final of neither the French Open nor Wimbledon, his maximum number of Big finals per season was 5 and he didn't have a single successful Big Title defence. Between 2011-2016, Djokovic averaged 3 Grand Slam finals per season (2014 was the only year in which Djokovic reached less than 3 Grand Slam finals). He reached 4 finals each at the French Open and Wimbledon, won the former once and the latter thrice. He averaged 9 Big finals per season and successfully defended a Big Title on 16 occasions. The difference between Djokovic's results before 2011 and between 2011-2016 is also rather clear.