How young did the person start?

By the time he was in the sixth grade, Larry [Summers] had created a system to calculate the probability that a baseball team would make it to the playoffs in October based on its performance through the Fourth of July.  In 1965 the Philadelphia Bulletin described Summers as the most qualified eleven-year-old oddsmaker in baseball.

That is from the new and very good Jon Hilsenrath book on Janet Yellen.