why do you care about youth - you get two players that are 2 stars and 3/4 stars - the better one cost you tokens to sign him. If you promote he will be 3 stars, and he doesn't develop as quickly as you need him to when you promote, if you sell him you lose money - you can get more cash from the bank with the same amount of tokens that you pay to sign the young player.
Get the 2 stars and sell him quick and let the other one go. That's what I do every time they come in my team....
Just a suggestion - you do what you think is worth for you...