I understand that the game has changed, but it had changed in the seventies from the twenties, it will always change.
However, at the same time, human beings have changed as well.
You point out diet back in the sixties, but fail to acknowledge how young people today benefit from every improvement along such lines in the modern era, and it shows as they are far stronger and fitter athletic specimens than their forefathers.
Everything is thus equal.
I think that during WW1 the average working man in the UK was about 5'6"
I can't be sure about that figure but it was significantly smaller than their sons who would come after.
Funnily enough, underage drinking amongst the working class back in the sixties was endemic.
I suspect most young footballers were as keen as their senior pros in that respect, so again, all things being equal.