The developing athlete has a big window of adaptation that is why if in the beginning stages of training if they do anything they get big improvements. It is almost like the more they do the better they get. The anything can be completely off the wall, but because they are an open book there is vast potential for improvement. The problem then becomes where do you go after you make those big initial gains? Too many athletes as they develop keep doing what got them there when the window of adaptation was huge, but as they grow and develop the window of adaptation begins to close. Because it gets smaller it is imperative that the work become more specific, directed and intense. I have found that it is tough for the athlete to accept that. They are often caught in an activity trap where more is better with a lot of fluff in their programs that could be replaced with more specific directed training. For the male athlete the window is usually wide open roughly at chronological ages 14 to 18, depending on whether they are an early or a late developer. For the female athlete the window is usually open earlier, generally ages 11-12 to 16, beginning just before the start of menstruation. For the athletes at advanced training ages and the masters athlete it is important to remember that pathways once opened can always be reopened. If you doubt that look at how stroke patients are able to come back to full function.
Honor and respect the window of adaptation concept, but do it by giving the younger developing athlete a sound foundation. Do not just throw junk at them. Teach them a good routine of training and educate them on the why of what they are doing. Even though the window of adaptation is wide open it does not all have to done in one year. Time is on your side.