To revisit this idea of player's maintaining a high level later into their 30s, the cut-off would really have to be AFTER Roger's generation. Consider that most of his best peers all declined in the usual age range, some even a bit young. Here are the best players of Roger's generation (born 1979-83), with the year-end age in which they were still either top 10, or won a big tournament (Slam, WTF, Masters):
Federer: 2002-17+ (age 21-36+)
Safin: 2000-05 (age 20-25)
Ferrero: 2001-03 (age 21-23)
Hewitt: 2000-05 (age 19-24)
Davydenko: 2005-09 (age 24-28)
Roddick: 2002-10 (age 20-28)
Nalbandian: 2003-08 (age 21-26)
Coria: 2003-05 (age 21-23)
Ferrer: 2007-15 (age 25-33)
As you can see, of the nine best players of Roger's generation, he's the only one still at or near prime form, and only he and Ferrer (2 of 9, or 22%) maintained a prime form into their 30s. Now a few others of his generation maintained a prime-ish form into their 30s--namely Feliciano Lopez and Ivo Karlovic--but those are lesser players.
Now if we look at the next generation, born 1984-88, we have a different picture.
Nadal: 2005-17+ (age 19-31+)
Djokovic: 2007-17+ (age 20-30+)
Murray: 2008-17+ (age 21-30+)
Wawrinka: 2013-17? (age 28-32?)
Tsonga: 2008-17? (age 23-32?)
Berdych: 2005-16+ (age 20-31+)
Cilic: 2014-16+ (age 26-28+)
del Potro: 2008-13+ (age 20-25+)
Soderling:2009-11 (age 25-27)
(It is interesting to note that both generations have nine players that I would consider 1st or 2nd tier players, or elite or near elite; in the younger generation, the next best group include players like Gasquet, Monfils, Isner, Almagro, Simon, etc, and in the older generation the next group includes Robredo, Gonzalez, Verdasco, Blake, Youzhny, Lopez, etc - all of whom are pretty much the definition of 3rd tier, to varying degrees).
So in the younger generation, we have six players - at least - who seem to be maintaining prime, or near-prime form into their 30s. Now maybe this is helped by a weaker younger generation - I think it is. But I'm not sure to such an extent that these guys wouldn't still be in the top 10, which is the easiest of the criteria for prime that I set forth for this post.
The older generation also has some older starts to the prime years - with Tsonga, Soderling, Cilic, and Wawrinka all beginning their prime years in the year they turned 23 or older. In the older generation, you had half that - Ferrer and Davydenko.
These are pretty small samples to make definitive statements, but I do think they at least give us a sense of an aging trends. What they do tell us is that more elite players are entering AND exiting their primes later, but what they do not tell us is whether truly great players are peaking later. Again, we're probably going to have to wait until "Generation Zverev" comes more fully of age.
