This is the in-line citation given by the official help page.

Young players advance very quickly while older players advance slower. All players improve much faster if they participate in matches; therefore, we recommend that you rotate your starting lineup in order to give all of your players a chance to improve.
I'm not asking you to trust what the developers says about the game, I'm asking you to find out if this is true.

A token or non-token youth have different base training value yes, but there are other factors that affect it, lack of pitch time is one.

A non-token youth will surely gain more than 10% for stretching, only if he played enough matchs. Even a token youth train extremely slow if no match is played, there is definately a hidden factor present.

A player who never played train extremely slowly, and this is a repeatable, verifiable characteristic. It is a solid prove that the above quote is true.