As an aside...One thing we should look for is that while it seems tier 2 and 3 players like Ferrer and Anderson, respectively, seem to be peaking later, more in their late 20s to early 30s, we don't know if the same is true of the tier 1 players - meaning, the Big Four. Stan is an interesting exception in that he was a tier 3 player (top 20ish) who became a tier 1 player, almost over night (Well, not exactly - he was looking like a tier 2 player in late 2013 before winning the AO in early 2014). The first Slam title seemed like a fluke, that he was more of a late-blooming Berdych or Tsonga (tier 2) who got lucky, but the second Slam cemented that he's a step above these guys and closer to Andy.
Stan has played his best tennis at age 28-30, with no sign of declining. Ferrer, Lopez, Anderson, and others are playing their best tennis, even a bit older (although I think Ferrer is slightly reduced in 2014-15 than he was in 2012-13). Roger, despite still being great, showed a more standard career pattern: peaking at 22-27, declining to a lower plateau from 28-31, then after recovering from his disastrous 2013, another slightly lower plateau at age 32-34. As remarkable as he is, it is still a relatively normal pattern - it is just that, as someone said, he came from so high so that a "plate career plateau" is still Himalayan. Couple that with his extraordinary work ethic and health and you have a 34-year old who is #2 in the world and looks like he could remain a top 5 player for a year or two yet.
But what we don't know is how Djokovic, Nadal, and Murray will look in a couple years. Rafa is clearly in decline; best-case scenario and he comes back with a slightly lower late career plateau, like Federer over the last six years. But we don't know yet if Novak and Andy will decline at that time as well, or if they will be able to do what other, lesser players have done - have an equal or higher plateau at age 28-32 as the traditional peak years of 22-27. Who knows, maybe we'll look back at this US Open as the first sign that Andy was dropping a half-step, like Roger's 2010 Roland Garros when he went out in the QF of a Slam for the first time in six years.
So to summarize, what I'm trying to get at is that for whatever reason, it seems that tier 2 and 3 players are more prone to late careers peaks than tier 1 elites are. I don't know exactly why this is. Maybe it is because they aren't elites, that they are more inspired to do what it takes to take their game up a notch - like Agassi being a better player in his late 20s and early 30s because he partially wasted his peak years to partying and such. Certainly Stan Wawrinka is setting a new standard, and is a hope and inspiration for every player in their peak years who hasn't won a Slam. But we can look at a Dimitrov or Nishikori or Raonic, or even a Gulbis, and not be able to assume that they will take the normal age 28ish decline. Maybe it all clicks for Grigor when he's 27 or 28 and he has a Stan-like or better late career peak.
