Lets just get this out of the way, even though this article is specifically about developing athletes everyone should be stretching! OK now, as an athlete who is striving to develop through intense training your body is going to be stressed in all sorts of ways and injury is the last thing anyone wants to happen. Unfortunately just that happens all too often and young talented athletes are left sidelined for what can be several months at a time. As someone who has suffered a severe injury before, I along with all of us at RSC want to help kids in every way we can to prevent this.
Before any activity weather its a game, practice or weight training stretching should always take place both at the beginning and the end. This will prepare your body for the movements you're about to do and lower your risk of injury.
So your growing larger in size constantly, training in your sport and possibly training your body to be stronger with weight training. The stronger your muscles get the tighter they also get, think like a rubber band. Every muscle in your body is meant to pull any two points on any two bones closer together. Each can only be stretched so far before "failing". Muscle failure in a way means injury (broken rubber band). The muscle failed to keep its two ends close enough together so the muscle was damaged and this results in injury. When developing athletes are getting stronger by the week those same muscles get larger and tighter (bigger rubber bands). After a period of time the athlete may over-reach or slip just one time and exceed the stretch handled by that now shortened muscle.
Every day of training those limits of your body are tested and any day no matter how intense the level of competition you are at risk of injury. Try to begin and end any exercise with a good 5 minutes of stretching along with doing it on your own here and there. Do this and you'll be doing everything in your power to keep playing the game you love for a long time with no unexpected setbacks.