That high school hoops phenom who plays like an NBA pro? The sixth grader who's already asking questions about organic chemistry? They're both precocious — meaning they're way beyond their years in skill or knowledge.
When you look at the Latin roots of precocious, it all makes sense. When pre (meaning "before") joins coquere (meaning "to ripen"), you have something that is ripening prematurely. And in the case of precocious, you are usually describing young people who have some adult-like quality about them. Maybe it's their vast vocabulary, maybe it's their ease with Calculus, or maybe it's just applying lipstick.