According to Elias

Washington Nats rookie outfielder Bryce Harper hit his first home run on Monday at the age of 19 years and 211 days. He’s the youngest player in Expos/Nationals history to hit a home run, breaking the record held by Gary Carter since Sept. 28, 1974, when he was 20 years, 173 days old. And he’s the youngest to hit a home run for a Washington team since Harmon Killebrew did it on Sept. 25, 1955 at 19 years, 88 days old. Pretty good company I would say.