Washington's 19-year-old outfielder is one of baseball's three percent -- the estimated portion of position players who have made it to the Major Leagues for at least one game as a teen since 1901, according to data from Baseball-Reference.com. Pitchers make it ever-so-slightly more often.
But playing in one game like Joe Nuxhall did at age 15 nearly 70 years ago, or even 40 like Mike Trout did last season, is a lot different than going out for 154, the record number of games by a teenager, set by the White Sox's Bob Kennedy in 1940. Harper, who debuted on April 28, is already at 30 games, and he could be just the 15th teenager to play enough that he qualifies for a batting title. Ken Griffey Jr. did it last in 1989.
"I didn't expect him to be here as quickly as he is here," Nationals manager Davey Johnson said. "I didn't expect him to look as comfortable as he looks up here. I think he is very relaxed. He is having fun, he is expressing his talent."
What promises would a full campaign as a teen bring for Harper's future? Well, none, but Ty Cobb, Al Kaline, Mel Ott and Robin Yount are in that group of 14, and just one of the 14 played fewer than 10 seasons in the Majors.
Harper entered Thursday hitting .274 with four home runs in 129 plate appearances.
Teen batting-title qualifiers
Ott put together an offensive season in 1928 that would likely stand as the unquestioned best by a teen if it weren't for Tony Conigliaro's 1964, when he hit a teenage-record 24 home runs -- that's without qualifying for the batting title -- for the Red Sox. Both were 19, just as Harper is, and both reached the qualifying threshold. Today, that threshold is 3.1 plate appearances per team game.
The world knows Harper is young, the world knows Harper is incredibly talented, the world knows maturity is the key word in any discussion about a kid barely old enough to vote playing at the highest level.
One Major League scout said it was clear that Harper, who had the hot-head label in the Minors, had grown up in just the past year. Harper appeared calmer than his general manager was after Phillies pitcher Cole Hamels plunked him this month.
"I was with the Mets [front office] when we had Dwight Gooden," Mariners general manager Jack Zduriencik said. "Having lived in the Texas-Oklahoma area, having watched Pudge Rodriguez come to the big leagues, Sammy Sosa, guys like that. A-Rod. It takes superior ability ... quite frankly [mentally and physically], both. When guys get to the big leagues, they're 19 years of age, which is very, very rare. It says something about how they're headed for a super career."
Johnson not only managed a teenage Gooden with the Mets in 1984, but a teenage Jose Oquendo the year before, and to Johnson, "[Harper is] much more driven than anyone I've seen."
If Harper were to keep up the pace, his .523 slugging percentage would be just one point below the mark Ott finished with in 1928. Of teens with at least 80 games played in a season, only Ott and Conigliaro (.530) would keep Harper company above .500.
But even if Harper's numbers drop or he doesn't reach enough plate appearances, there are others from whom he can draw experience. Kaline, who did qualify for a title in 1954, had just four home runs and a .276 average. The next year, he actually won the title at age 20, with a .340 clip.
"He should rely on anybody who is willing to talk to him and give him good, sound advice," Kaline said when he saw an 18-year-old Harper. "The one thing I would tell him is just go and keep your mouth shut, do all the little things that are required. Go over and above what you are asked to do. Get to the ballpark early, listen to the players as they talk, listen to the coaching staff and manager, and just focus on baseball. Don't focus on the outside world and what people are saying about you."
This much is certain: It's only the start.
"When I was 19, I was in the New York-Penn League. I was a baby," said Red Sox rookie third baseman Will Middlebrooks, who is 23, made his big league debut earlier this month, and played with Harper in the Arizona Fall League.
"I was real immature, not only in baseball but life in general. I can't imagine being in the big leagues. ... Harper's still a kid. When he fills out and he figures everything out, he's going to be dangerous. It's going to be crazy."