Don't bother signing any youth player that cost less than 21 tokens. For them to be an integral part of your squad you need to make sure you've got around 100 greens to speed train them up to scout level.
I wouldn't bother signing anyone after matchday 10 either. A lot of people on here don't rate the youth system but i've got 3 players in my starting line up from it and another 1 on the bench. There are a couple of things that really annoy everyone about it if they enter the squad on matchday 1 they turn 19 on matchday 2 that's a con. The positions you really want are the positions that more than likely you'll never get.
If it's something you really want to do save all your greens and just accept that even if a 24 token youth pops out in the same position as your favourite player get him as after you've trained him he'll be great for years and the 24 tokens won't seem that expensive 5 seasons later.