If a 18YO has the same Q and the same price as a 26YO, he will not be a fast trainer.
If ypu want to buy/train youth, you can buy them when the new season starts, after the refresh, the transfer list if usually full of 18-20 YO, from bot teams.
They are all very expensive, and train very fast.
Level 1 - League 3rd place (started late, in the season)
Level 2 - League 2nd place, CL 2nd place (both due to troll results)
Level 3 - League 1st place, CL 1st place, Cup (lev 6) 3rd place
Level 4 - League 1st place, CL 1st place
Bought 28 Tokens, referrals 32 Tokens, won 3 Tokens.