Few 19-year-olds have ever done what Bryce Harper did for the Nationals this season, a fact that was repeated over and over during the course of his award-winning rookie campaign. All along, there was an assumption that Harper had only scratched the surface of his immense abilities, and that he would be poised to take his game to even greater heights at age 20. That assumption was based not only on Harper's individual case, but on the history of baseball itself, which features plenty of examples of young players making a significant leap once they turn 20. How much better, though, can we expect Harper to be in 2013 than he was in 2012? By analyzing the career progressions of other prominent teenage ballplayers, we can start to get an idea. There were 13 players in major-league history who received at least 400 plate appearances in their "age 19" season prior to Harper. That list includes Hall of Famers Mel Ott, Robin Yount and Al Kaline, likely future Hall of Famer Ken Griffey Jr. and teenage phenom Tony Conigliaro (who might well have made it to Cooperstown had his career not been derailed by injury).