The word ‘potential’ can be misleading. For various reasons, an athelte may be at the peak of their genetic ‘potential’ the first time you see them, even if at a young age. Consider Olga Korbut. She was destined to peak at a very early age. Had her genetic makeup been one that kept her small and petite, she may have continued on for several years and continued to improve.
You can’t predict the future. There are an infinite number of variables that go into performance, some more important than others, and its incredibly difficult to predict any of them, much less all. An experienced coach can be right much of the time, but the best coaches and scouts are fooled much of the time.