Pinpointing the single most crucial stage in player development is never clear-cut. Every player develops at a different pace and each youngster has a unique set of priorities at various times of their lives. There is, however, a few constants throughout the lives of these players, and one of them is coaching. A coach, good or bad, can have a tremendous impact, good or bad, on a player, some lasting and some fleeting.
-excerpt from US Soccer Players website