It's always a difficult decision to make. Depending on what level you are, money may be a factor too. At least the market value of the youth players tend to be somewhat equal to what a bank investment would get you for the same amount of tokens. So it's often worth it to sign the player and see if you will use him or sell him afterwards. The mental aspect of releasing your youth player and having someone else sign him as a free transfer is also slightly upsetting.
18-24 tokens is a big chunk if you only have 35 tokens per season income though, so nobody can make this choice easier for you I'm afraid.