I don't think this is true either. Took a while for Rod Laver and Ken Rosewall to find their way and they dominated way into their 30s. Rosewall played effectively into his 40s.
These kinds of things are likely cyclical and have more to do with circumstances related to touring and different motivational factors (e.g., in Rosewall's and Laver's case, as well as Pancho Gonzales's, they were financial).
Overall, there's something to said about the way the standardization of the ATP tour (beginning with 1990) contributed to improving the mental health of tennis players and extending careers. 1970s and 80s tennis was a pretty crazy time, with complicated and unpredictable touring, and heavy recreational drug use among players.
The real extending of careers didn't arrive to much later than that, however. Back in 2013, I did a piece of research into the average age of GS winners in the period between 1987 and 2013. Out of a sample of 106 GS tournaments, only 15 were won by players 28 or over, five of those by one player, Andre Agassi. If we eliminate Andre, only 10 times did it happen in a quarter century plus. With the exception of him, it was an extremely rare feat for anyone older than 27 to win a GS until the past eight or nine years.
I also got curious about the average age of Grand Slam winners in this sample, so I did two compilations: one an average for the ten years previous to Federer’s first slam, 1993 to 2002 (Why those years? Because once Federer and Nadal appear on the scene, there’s not much of a sample to test in terms of other players winning Slams). I also did a sample including all years from ’93 to 2013,
excluding the big fish, Federer, Nadal, Sampras and Agassi, so as to get a better sense of how mere mortals do.
In the ten year sample, previous to Federer’s first Slam in 2003, here are the average ages of Grand Slam winners for each of the four majors in this ten year period (this includes Sampras and Agassi’s titles during this period):
Australian: 26.2
French: 24.8
Wimbledon: 25.3
US: 24.4
Overall average of Grand Slam winner: 25.1 years of age
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Excluding Federer, Nadal, Sampras, and Agassi, 1993 to 2013, the average age of the other 26 Grand Slam winners in this period is almost exactly 24 years of age.
The breakdown for this group (again excluding the Big Four) is:
30+ year olds: 0 (Petr Korda. ’98 Australian Open winner, missed by a few days)
28/29 years old: 3 players
26/27 years old: 5
24/25 years old: 7
22/23 years old: 5
20/21 years old: 6
Obviously a lot of reasons influence this change including conditioning, equipment, diet, three seemingly superhuman specimens as role models, and changes brought about by the ATP over the years. But it is a dramatic change. So I do think it is safe to say that many players now peak later than their earlier counterparts.