Here is one of the rare studies on stamina giving the necessary % to retain player's stamina level (by age of player).
My rating : Useful but incomplete.
Here are the results of a study on stamina giving the potential reachable level of endurance in function of player age and training percentage choosen.
Unfortunately I have no information on the origin and quality of the study, but the results seem very consistent at first sight.
My rating : a must.