At 19, Harper straddles the line between late adolescence and full-blown adulthood. His sheer skill as a baseball player makes that easy to forget, but Harper’s age will color every aspect of his mission to make the Nationals’ opening day roster.

With less than 600 professional plate appearances, has he developed enough? After he tore a hamstring last August, can his body handle a full major league season? Fully embracing his generation’s proclivity for personal exposure, can he display the professionalism Nationals management demands?

He’ll try.

“I’m going to work as hard as I can, keep my mouth shut and play,” Harper said. “I’m going to make their decision hard as much as I can. I want to be up here. I want to play, and I want to play in D.C.”

At the outset of the winter, Manager Davey Johnson told General Manager Mike Rizzo “he should definitely have an open mind” about putting Harper on the team. After their winter acquisitions, “he’s still in the mix to have an opportunity,” Johnson said.

Johnson and Harper first met when Johnson gave him an award at a showcase event when Harper, then 15, beat out the best 16- and 17-year-olds in the country. Johnson watched as Harper tried to make the Nationals last year, at 18, even when all evidence, rightfully, suggested he would not.

“I got the feeling it wouldn’t have been overpowering to him mentally, because his whole life he’s been competing with guys older than him,” Johnson said.

Harper’s belief in himself far exceeds the typical ballplayer. Take Anthony Rendon, this year’s sixth overall draft pick. Rendon played three years of college baseball at Rice and turned 21 last June.

“I’m more reserved,” Rendon said after settling in for his first day of spring. “I’m not like, ‘Oh, yeah, I’m shooting to try to go straight to the bigs.’ I’m going to take my time and try to get used to this lifestyle.”

Before Harper split for the day, he spoke to a small pack of reporters, wearing a hat bearing a logo for the apparel company Undefeated. “Beckham and Kobe wear it,” Harper explained. He freely admitted it would disappoint him to start 2012 in the minors.

“Even last year, I was disappointed,” Harper said. “I came here, and I wanted to make the team last year. This year, I’m trying to come here and make the team. Hopefully, things work out and we won’t have to talk about me going down to the minors.”