Jordan Spieth waits behind a green during the second round of the 115th U.S. Open Championship at Chambers Bay. (Harry How/Getty Images)

Jordan Spieth is the only 21-year-old who should be carded just to make sure he isn’t a whole lot older.

When you’re 21, in the lead at the U.S. Open and you make a double-bogey on a hole that you think is “dumb,” it’s supposed to frazzle your young golfing mind.

If that hole is designed to favor “the 10, 12 guys that can fly it 310 yards” but presents an unfair challenge to normal-length players like you, then you might get furious, go all scatter-shot and crumble into a heap of self-pitying Open anguish.

Instead, Spieth listened to the advice of his caddie not to start runnin’ hot and birdied the next hole in the chamber of horrors here at Chambers Bay on Friday.

PGA Instructors Brent Zepp and Ryan Young offer golf tips and techniques for the Chambers Bay golf course. (Chambers Bay Golf Course)

When you’re one shot out of the lead on your final hole of the second round and, suddenly, a playing partner collapses from vertigo and lays on the ground for 10 minutes, surrounded by medical personnel, it’s supposed to rattle your immature nerves.

Seriously, when the 10th-ranked player in the world just drops like he’s been pole-axed right next to you, then when he gets up and crumples to one knee again and finally plays out the hole staggering on rubber legs, it should get in your head.

Spieth organized the crowd around fallen Jason Day to give him room to breathe, shooed away cameras and, after a 15-minute delay, ended his round by sinking a 10-foot birdie putt for a 67 and a 5-under 135 that put him in the clubhouse tied for the lead.

“That was one of the best birdies I’ve ever made, given all the situation,” said Spieth, who is tied with Patrick Reed with a one-stroke lead atop the leader board.

Two classic golf distractions, two birdies for the young Texan, who is of average height, average build, average pro-golfer length, yet absolutely exceptional quality in every other way golf can measure.

You can’t adopt him. He already has parents. (And you would be in line behind me.)

Spieth, who won the Masters in April, has a chance to become the sixth player, along with Arnold Palmer, Ben Hogan, Jack Nicklaus, Tiger Woods and Craig Wood, to win both the Masters and U.S. Open in the same season. Bunch of bums.

You would think he might feel pressure or look for bad omens, like a collapsing partner. But Spieth is only 21 in one place — on his birth certificate. In every other sense, he has a maturity that seems ageless.

A public celebrity life as a world-stage star athlete at such an age is like walking down a street with a hundred open manhole covers. Potential pitfalls, some small but all annoying, are everywhere. Yet Spieth knows where to step.

As he played the 18th hole (his ninth of the day), Spieth was in bunkers twice on his way to that double bogey and fumed, into an open TV mic, “This is the dumbest hole I’ve ever played in my life.”

Kids aren’t supposed to criticize the finishing hole at the U.S. Open. You’re knocking the architect (Robert Trent Jones, Jr.) and the executive director of the USGA (Mike Davis). But Spieth managed it perfectly. And he’s right.

In a stretch for novelty, the first and 18th holes can both be played as a par 4 or 5. So each day, the tees get jacked around, and one is one thing and the other is the other.

“The 18th, as a par 5, doesn’t make much sense. Of course, when I didn’t hit the right shots, it’s going to make less sense. If microphones are going to pick up [what I say], then they’re going to pick it up,” Spieth said. “I’m not going to put a smile on and be happy with the way I played the hole. So I am who I am.

“The hole doesn’t make sense, because you can hit it down the left center of the fairway and still end up in the right bunker in trouble,” he continued, noting the sloped fairway. Those 10-12 bombers can fly over the trouble and “play an entirely different hole. . . . For anybody else, you have to hit in a five- or six-yard area. . . . The tee shot should have been moved up. But I’m not the one putting the course together.”

Spieth’s choice? A drive through a needle’s eye.

“I wasn’t going to hit 3-iron into a 550-yard par-4, then hit 3-wood. So all in all, I thought it was a dumb hole today,” Spieth said.

Then came the special Spieth twist. Most players, even top ones, get hung up on injustice — if it afflicts their chances.

“We’re going to play it from there again, so I’ve got to get over that,” he said. That meeting might be on the 72nd hole Sunday with Spieth in or near the lead.

Several of golf’s greatest players won majors at 21 or 22. But few, if any, seemed to know themselves so well, and speak their minds without seeming peevish as Spieth already can. Tiger Woods finished with the second-worst score in the entire field (80-76).

Were players discussing Woods’s plight?

“It’s a U.S. Open. We have to maintain an insane amount of focus on ourselves,” Spieth said. “But, sure, it’s Tiger. Everybody is aware. Everybody hopes that he’s back and contending soon. I’ve certainly enjoyed my time with him playing practice rounds and learning from him. Wish him the best.

“But, no, there’s really no murmur.”

How do you balance respect without saying that you actually hope he wins — which means that he would beat you? How do you appreciate Woods’s generosity with advice but acknowledge that, for now, he’s not even a murmur?

Spieth seems to find his way with a spontaneous candor that doesn’t contain malice.

When Day chipped in from 60 yards for a birdie, Spieth reacted like a fan. “I screamed . . . as if I was sitting in the gallery,” he said. “It was a cool shot to see.

“Probably wouldn’t scream if he did it on Sunday and we were tied, but it was nice today.”

In normal human development, 21-year-olds are only supposed to be partially formed, ill-suited to seeing all the sides of complicated issues, and at times they’re just jerks. Years later, that all gets sorted out, and we meet the person they become.

Spieth has shattered that view. Many say he’s just what U.S. golf needs. He might not be a bad influence on American manners either.

Do we all have a period in our lives when the best in us shines while what is darker stays hidden?

How long can such times last?

Perhaps we can hope that Spieth is an exception, a man who never grows up too much. Since he already has.