I recently put together a ranking chart of every #1 player according to TennisBase.com, for all of tennis history (from 1877 to the present). One of the most interesting takeaways was how the age of greats had changed over time. Players remained elite for much older before the Open Era, especially the further back you go. Consider...
*Bill Tilden is well known. Born in 1893, according to TB he was #1 in 1932 at the age of 39 and #2 as late as 1935 when he was 42. He remained a top 20 player until 1945, at the age of 52!
*Josiah Ritchie (b. 1870) is a forgotten great, mainly because he didn't win any Slams but won tons of other tournaments and was one of the top players of the first decade of the 20th century when he was in his 30s, the top player in the world in 1904-05 and 1908-09, according to TB. He was ranked #4 in 1920 when he was 50 years old and in the top 100 as late as 1927, when he was 57 years old!
*More recently, Pancho Gonzales (b. 1928)--the best player of the 1950s--ranked in the top 10 into his early 40s and in the top 40 through 1972, at the age of 44.
There are many more such examples of pre-Open Era players remaining top into their 40s and very good players as late as their 50s. Ken Rosewall, the last great player from the pre-Open Era won his last Slam at age 37 and was a competitive player until his mid-40s, in the late 1970s. He was the last great to be that good into his 40s, although Jimmy Connors came close a couple decades later when he finished 1992 #84 at age 40.
The tour grew much younger in the 70s and 80s, both in terms of decline and retirement age but also when players were breaking through. Think of 17-year old Slam winners Wilander, Becker, and Chang, and simply a ton of teenagers ranked in the top 100 in the 80s and early 90s. There were young Slam winners before (Rosewall won his first two at age 18 in 1953), but the age of players grew over all younger and with fewer older players. Most of the top players of the 70s-90s were retired or declined by the time they turned 30.
Now I don't think we're going back to an era where top players continue playing into their 50s, but we may be seeing elite players remain elite into their 30s more frequently, and even some playing as late as 40. Of course the question is whether a 38 year old Djokovic will be able to compete with a 28 year old Zverev or a 24 year old Aliassime. It may be that players played much longer back in the day because tennis wasn't so centralized around the ATP tour and Grand Slams like it is today. There wasn't even a proper pro tour until the 1920s, I believe, and of course the Open Era (1968) brought the amateur and pro tours together. So it could be that whatever changed the age range of players was directly because of the Open Era and the centralization of the tour.
Roger Federer has remained an elite (top 5) player into his mid-30s. Agassi was still top 10 in 2005, at age 35, reaching a Slam final (losing to Federer). Before him you have to go back to Connors, and before Connors, Rosewall and Laver in the 70s. So elite players in their mid-30s have been quite rare in the Open Era, as most have declined significantly in their early 30s or even late 20s. It seems that Andy and Novak are aging well as they are both approaching 30 and #1 and 2 in the world. What we don't know is how they'll look in two or three years, especially as it looks like there are finally some good young players coming up.
Again, I don't see why a 35-year old Novak or Andy couldn't remain a top 10 player. But the question is whether they'll be good enough to win Slams over peak Thiem, Kyrgios, Zverev, etc. Or how long Federer can remain top 5 form, or even if he can return to the top 5 at age 35-36 (he certainly looks like he can at least be top 10 for a year or two).
