Herios, I'm not sure how "current facts are against [my] theory." What I think we can't tell yet is whether the age range has shifted back, as you say, or that it is more that we're in a talent lull (or, as I think likely, some of both). But I don't think we can say that it is just an age shift simply because there's no great player under age 25. There have been other times in tennis history in which there was a lack of top young talent - take the late 90s, for instance. Sure, there were some excellent young players but no true greats until Federer came along.
I'll see if I can create a chart that expresses this.
Here's one thing we can do relatively easily. Here's a list of the birth years of every player of the Open Era that has won four or more Slams - and within the Open Era. Now to some degree the criteria of 4 Slam victories is relatively arbitrary, but I think it is a decent simple way to define all-time greatness; at the least I think we can say that the 4-Slam winners (Vilas and Courier) are on the lower end of true greatness, and that the 2-3 Slam winners (e.g. Kuerten, Safin, Hewitt, Bruguera, Kafelnikov, Rafter, Kodes, Ashe, Smith, etc) all fall a bit short of true greatness.
1987 Novak Djokovic
1986 Rafael Nadal
1981 Roger Federer
1971 Pete Sampras
1970 Andre Agassi, Jim Courier
1967 Boris Becker
1966 Stefan Edberg
1964 Mats Wilander
1960 Ivan Lendl
1959 John McEnroe
1956 Bjorn Borg
1952 Jimmy Connors, Guillermo Vilas
1944 John Newcombe
1938 Rod Laver
1934 Ken Rosewall
As you can see, the greatest gap is between Sampras and Federer - a span of ten years, which is the only gap longer than 5 years between Connors and Djokovic. If we go back to Rosewall, we get the following gaps:
4, 6, 8, 0, 4, 3, 1, 4, 2, 1, 3, 0, 1, 10, 5, 1
That's an average of 3.3 years, with a range of 0-10. Let's take a look at the whole span, which would be 1987 (Djokovic's birth year) to 1997 (juniors turning 16 years old this year). Here are some of the better players (that we know of) born in those years:
1987 - Andy Murray, Sam Querrey, Jeremy Chardy, Thomaz Bellucci
1988 - Juan Martin Del Potro, Marin Cilic, Alexandr Dolgopolov, Ernests Gulbis
1989 - Kei Nishikori, Benoit Paire, Martin Klizan, Donald Young
1990 - Milos Raonic, Jerzy Janowicz, David Goffin, Yevgeny Donskoy
1991 - Grigor Dimitrov, Andrey Kuznetsov, Denis Kudla
1992 - Bernard Tomic, Ryan Harrison, Jack Sock, James Duckworth
1993 - Jiri Vesely, Dominic Thiem
1994 - Lucas Pouille, Filip Peliwo, Luke Saville, Christian Harrison
1995 - Nick Kyrgios
1996 - Christian Garin, Gianluigi Quinzi, Noah Rubin
Andy Murray still has a chance of becoming a 4+ Slam winner, but its hard to imagine any of the other players in the 1987-89 birth years accomplishing that. There's a pocket of possible players in 90-91, namely Raonic, Janowicz, and Dimitrov, that have outside chances. After that, who knows? But it does seem that the next great player - if we define "great" by 4+ Slam wins - was born in 1993 or later.