Isnt it obvious? Tennis has become extremely physical. Only athletes with peak endurance and strength can compete. And it takes time to reach that level. No 19 year old is winning the tour de france any time soon either.
This is completely nonsensical and the analogy of the Tour de France is completely irrelevant. Tennis is not a sport which relies so heavily on Lactate concentration or VO2 max levels. Cyclists of the Tour de France do not rely on agility, flexibility, strength and power as much as tennis players do, which favour the younger athlete.
In the past, teenagers and those in their very early 20's (21 or younger) have exhibited excellent displays of physicality, as recent as the likes of Andy Murray, Novak Djokovic and Rafael Nadal and it's no coincidence that they top the World Rankings. In 2007, people noted that Andy Murray wasn't fit enough, physically and mentally. A drastic development occured over the course of 4-5 months, from late 2007 into the early months of 2008. He put in the work, why can't those of today? There is absolutely no excuse.
We can go back to the late 90's and early 2000's, when there was an influx of young talent approaching with the likes of Tommy Haas, Gustavo Kuerten, Lleyton Hewitt, Juan Carlos Ferrero, Marat Safin, Andy Roddick, Roger Federer and then lower tier players such as Juan Ignacio Chela and Andreas Vinciguerra displaying the goods from an early age. For example, Kuerten was winning Roland Garros on a slow clay court bettering opponents in multiple 4 and 5 set matches, Roddick was winning back-to-back Masters Series events and a Grand Slam and Marat Safin was winning 5 setters against defending champion Gustavo Kuerten and Andre Agassi at Roland Garros aged 18 and winning the US Open and becoming World Number 1 aged 20. You can give similar examples for all of these players relevant to their respective levels of ability.
Scientific research has largely proven that flexibility and agility are superior before the age of 24 years old and there is bundles of research to illustrate that cardiovascular levels can be easily
attained by 18 years old's to be sufficient and more so in tennis. This is without the topic of injuries, how younger players are far less likely to have suffered injury and undergone surgery leaving scar tissue etc. which can inhibit flexibility and cause further damage.
There is absolutely no reason why younger players cannot be physically efficient and only have to worry about polishing up on technique and other fundamental skills required. Largely to blame is the coaching and personal advisers that come along with academies and other coaching facilities which are extremely well paid.
I mean, look at Girgor Dimitrov, someone who must weigh a measly 78kg and seems to regularly struggle physically. Not for one moment am I saying that hypertrophy increases power, like many do on this board, but a certain type and amount of hypertrophy comes with regular resistance training for strength and more essentially power. I feel that these players such as himself, feel that they are progressing and doing enough as is, so why change? I say, how good could those players be if they were as physically able as Andy Murray aged 20? A hell of a lot better, you bet.