I guess this is more related for the old version... when, the players had chances of injury during the trainings, so was a simple advice.
Then I have to say that, I dont know, if, there's something internally that when one train hard after hard increase the risk of injury.... as some managers never trained hard and had a good amount of these...
Is like when you say, play with young and old players, I guess is more about effectiveness in performance&economic side, because you can have, some youngers, fast trainers who, can be the best Q. players of your team, same time, it is posible to find old players that with 5* are well programmed so they can give you a lot for a low cost if one know how to do a balanced team, and how search for adequate players.