From what I read on this debate, I think that people should be careful that the debate is not so much youngsters (teens are mentioned here for instance) vs oldies (>=30 years old),
but rather youngsters vs players with an "average age" (24-28)
The phenomenon of youngsters failing to emerge is much more visible than any phenomenon of old players having more success nowadays. Statistically, players who are 30 years old or more still appear to quickly decline, even though it appears less visible at the present moment because there was a high number of very good players in Fed's generation, and also a relatively high number of players who managed to better use some important parts of their potential with time (Ferrer, Fish, Melzer, Benneteau, Lopez, Youzhny, even Federer are in that case).
I say that especially because of that comparison with cycling or marathon which has led to the argument of the endurance of >30-years old guys.
Like Burrow, I don't think it's relevant in tennis.
But ... the question remains whether a 24-year-old guy may be more endurant and solid than a youngster, both physically and mentally. Also knows his body better ... and maybe uses doping better, because, sorry, when one speaks of the undeniable improvement of the doctors and physios' watch in tennis, I can't help thinking of the possibility of doping either.
Especially, as far as solidity is concerned and as I said, I'm surprised by the number of youngsters who have had big injuries in recent years. I gave a list a few posts ago and that list appears to be very long to me.
Of course, in that "endurance", one can't help thinking as well of the "mental endurance", that is the ability and motivation to work hard and constantly, as some said.
PS : Burrow spoke of Kuerten : when Kuerten emerged, he was already more than 20 years old.
Kuerten won Roland Garros at 20, so no.
And about the 24 year old vs. "youngster", carrying the potential to be more physically superior is also completely irrelevant. I'm not going to repeat myself, so I shall leave it at that.