The Washington Nationals outfielder has already been elected to two All-Star Games and won the 2012 NL Rookie of the Year award. He has become one of the top players in the game, and the world has taken notice.
The 20-year-old phenom could potentially still play for another two decades, but he isn't wasting time, as he's already tearing things up. In fact, he ranks third all-time for most home runs by age 21.
Harper's incredible performance got us at Bleacher Report thinking about the best players in MLB history who were 21 years old or younger.
Check it out.