Here is my take on this...

You get a 2* free youth player which normally a slow to average trainer and only worth selling on for some extra $ f you need it.

The 4* youth player is normally a fast trainer and we can sometimes see some speeds of 60/120/180 on all 3 training types on hard. If you have plenty of greens around to spare you will get a choice of what special ability you would like to give the player and any extra new position meaning if your prepared to do a little bit of power training with some greens it can actually be worth signing the 18 year old youth player since you will get many seasons from him. But bear in mind if your using TOKENS to power train then it would still be more effective to buy 4 or 5 scouts for the same cost of power training a 18 year old out the academy to a similar level as your best scout players from the scout list.