To give example, here in this list I see the 3 star GK should be a faster trainer, below him is 19 yo MC 4 star, only 1 player above him, also young player 20 yo 5 star. So if I needed a GK I would buy him and test him, also if he was 18 years old of course.
I would however never buy the MC. A 3 star GK of the same age is listed above him, so he is 100% going to be a slower trainer, not worth buying.

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