18 is 3rd best, 21T is better and 24T is the best, but some here will say that it is only a guideline that the highest token players are the fastest trainers.
Before you buy one of the youth players, always go to the transfer list and see if you can get an 18yo of that position and try to get the most expensive for less tokens than the youth would cost you, in this case 18T. If you can pick up the player on the transfer list for less than 18T then dont bother buying your youth, unless you dont have the money to buy it, then its a trade-off = youth for 18T and no money paid, or youth from auction for 5-10T and mucho $$$ paid. If you have the $$$, its better to buy from the auction list.