There's a lot of good advice in the Youth Academy thread, although it's so long at this point that picking out the nuggets of information that will help you won't be easy.
Kane offers good advice above, although there are also two variable attributes that directly affect the speed at which players train. Have a look and see if you can work out what they are, but the training page does make it reasonably obvious... as Kane said, your youths' maximum potential is indicated by their lowest fixed attribute excluding loyalty and you may see "red bars" appearing when a player has maxed out a particular ability- if your coaches are good enough to get him to that point.
Bear in mind that your squad size is getting dangerously low and you risk the sack if your squad size drops below 16 for an extended period of time.
Good luck
