They should start at 16 when you pick a position for them then at 18 they can play. They should have no score until they are 18 and come in +/-10% of players you currently field in that position. 4(?) should come through from the academy 2 to keep, 1 must be sold and 1 rejected. Perhaps the level of academy controls how many you can the number that come out and can be sold for recruiter points.