Well, that's why you need to plan accordingly and make realistic goals. If you do train an 18 year old then surely there's bound to be certain weeks in your schedule where you need to make a sacrifice for league or cup. I do have an 18 year old allstar player I drafted who I'm doing tiny bits of training but don't plan to make him anything special but a good backup. I'm willing to sacrifice some training minutes just to win. Just be smart when and where you play your trainees. ;)
Also don't start raging against me just because a few games haven't gone your way this season. I've had two teams (Slam and Urmummas) buy up before my game but they lost even doing so. Not my fault all teams aren't trying hard enough to win against me. But if they try, don't expect me to back down so easily. :p