A prodigy is someone who is so naturally talented at something that they become a master of that particular skill as a child — you can be a musical prodigy or a math prodigy. Mozart was one, writing symphonies and playing for kings when he was only five years old.
Prodigy is one of those wonderful words whose different meanings tell a story about how the meanings of words bloom over time. The word derives from the Latin prodigium, meaning "an omen or a sign of something to come." Prodigies are kids who often seem so talented that their success must presage even greater mastery, though, of course, the irony is that most prodigies peak in their youth.