Bryce Harper’s supernova start is full of “firsts” and “youngests” — as in, youngest National League player to start an all-star game and youngest to hit two home runs on his first opening day. Let’s also not forget: first 19-year-old to hit himself in the face with a bat he caromed off a dugout wall in anger, first 20-and-under to unsuccessfully slam-dance with two outfield walls, youngest to text his manager “play me or trade me” and, yes, youngest and most churlish to be thrown out of three games before 21, once per 66 games.

This isn’t a get-off-my-damn-lawn sermon to an uppity faux-hawked rebel without respect for his elders or authority; indeed, Harper has the exact amount of brash and cocksureness needed not to cross into arrogant-young-jerk territory.

But it is a way of saying that the same mayhem that makes Bryce Hustle impossible not to watch on a baseball diamond also has consequences.

Those consequences include lingering knee issues from inexplicably running into those walls; prolonged damage control to assure everyone you were “kidding” during your little frustrated-in-the-moment ultimatum to the manager; thick sarcasm after getting tossed in the eighth inning of a one-run game your team eventually lost in the 10th. Add all that up, and you’re not the only one suffering growing pains.

The Nationals are 31-26 when Harper plays and is not ejected. They are 17-22 when he is hurt or told to sit. During the remaining 66 games of this underachieving season, what they need more than even their young star’s unbridled aggression is his discipline.

The Post Sports Live crew debate what manager Davey Johnson should do with the Nationals batting order to kickstart the offense. (Post Sports Live)

Harper’s live bat, his discerning eye at the plate amid some of the free-swinging, undisciplined older guys on the club, his bolt-of-adrenaline demeanor from the dugout steps to the outfield — all of it makes Washington a much better team suited to catching the Atlanta Braves in the National League East the second half of the season.

But some of the rash judgment chasing after balls well hit; the I-know-I’m-right stubbornness that occasionally oozes out in the batter’s box when a pitch is framed just right or, in some cases, a flat-out strike; the defiant ’tude that takes over when things aren’t going right — that part of Harper’s game has to check itself before it wrecks all the good things he brings.

Part of Harper’s success certainly has to do with his hotheadedness, that vestige of a hand grenade in a batting helmet, topped off by that spiked, gelled ’do. The intensity is part of the package, what has so endeared him to Nationals fans. He doesn’t play as much as he ticks. He’s Kelly Leak from the original “Bad News Bears,” sans cigarettes and a motorcycle, a hellion whose force of personality and purpose really feels like he’s 20 going on 35 — especially when he’s got the playoff beard of a grizzled NHL third-liner working.

That fierce drive, Harper’s ability to put so much force and fire into his swing or his dead sprints in right field, is also one of the qualities that triggers historical comparisons to Al Kaline, Mickey Mantle and Ken Griffey Jr. at the same age.

His ability to harness that drive, though, is what worries.

It’s not a coincidence that Harper’s batting average has dipped nearly 100 points since he first hit a wall in Atlanta on April 29. He still leads the Nationals in on-base percentage, but it has dropped from .437 to .371 since his first collision with an inanimate object in the outfield. He was hitting .287 when he went on the disabled list. In the 14 games since he returned, his average has fallen to .264.

His injuries have forced him to miss 31 games and essentially killed any chance for his MVP candidacy, which looked good that first month. More important, they left the Nats’ anemic offense without one of its biggest bats at a time it was thirsting for runs. Davey Johnson has been cutting-and-pasting mostly unsuccessfully since Harper first went down.

The truth is, everyone in baseball and beyond wants to embrace Harper and will discard almost any flaw to do so; that’s how infectious his energy and talent are.

Look, getting thrown out twice in junior college games at 17 can be chalked up to youthful transgressions. Blowing a kiss toward the pitcher he homered off in Class A ball is, well, minor league, just like taking 10 sutures for throwing the bat off the dugout wall last May. Pointing the bat at Ozzie Guillen actually should be commended because Ozzie has done more to embarrass himself and baseball than anyone could cram into a career.

But when the St. Louis Cardinals give you grief for a crotch-grab after going down on strikes in the ninth inning of the deciding Game 5 of last year’s playoff series, when games are missed because of injuries that didn’t need to happen and innings are missed because of balls and strikes that didn’t need to be argued, it’s not merely the learning curve of a young player anymore.

Taken together, a pattern is forming: Every memorable stretch of play has a worrisome downside attached. And as good and wonderful as Bryce Harper is in so many facets three months before his 21st birthday, he might want to take a look at some of it before he’s a jaded veteran at, like, 25.

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