I think it mostly depends on how well the players sync up together. I've had young players rating improve exponentially while under the tutorage of certain guys, just mattered how well they got along. I'm sure some of that can be attributed to the natural growth of the players, but the tutoring certainly aided. My current striker in my Rangers game wouldn't be, in my opinion, half the player he is without Kris Boyd's helping him along at age 20-21. He'd still be a fine player, but I don't think Arsenal, Real, Valencia, et al would be knocking down my door with 25m+ bids for him. Thankfully he has no intentions of leaving anytime soon, so hopefully I can get another 5-8 years out of him.
Just make sure the players you link up have similar mental ratings; that'll pretty much ensure your youngster will get the most out of the experience. | 
