Kimberly Schroeder was happily single and childless, right up until the moment last year when Justin Bieber announced he was bringing his world tour to Charlotte.

Next thing she knew? “I was walking around the office, like, ‘So, does anyone have a kid I can adopt? Please?’ ” says Schroeder, 38, a Charlotte tax accountant.

Her co-worker Sheri Shaughness came through with 10- and 17-year-old girls, and together the four will be at Time Warner Cable Arena on Tuesday for a sold-out concert that will tax thousands of vocals cords and eardrums.

Most of them will belong to an ocean of tweens and teens; but look closer and you’ll spot older superfans in every section of the building, folks who have more in common with Bieber’s mom than with his ex-girlfriends.

They’re the ones who you’d assume are attending as chaperones – until you see their Justin Bieber earrings, or until you watch them shout every single word to “As Long As You Love Me.”

Or until you notice the custom-made T-shirt, like the one Sarah DesChamps plans to wear to the show that reads “Justin’s One Less Lonely Girl,” a reference to a hit song Bieber released when he was 15 years old. (He’s now 18.)

“I just think he is really cute. I know it’s weird,” says DesChamps, 34, a legal assistant who is driving four hours from Murrells Inlet, S.C., with three grown-up friends to get a glimpse of Bieber in the flesh. “I don’t care if somebody makes fun of me … well, my husband’s not that happy, but he’ll get over it.”

Growing popularity

To say Bieber is popular is like saying the South gets a little humid in the summertime. He was named the third-most powerful celebrity on the planet in 2012 by Forbes magazine, which reported last May that he’s sold 15 million albums. That was before his third studio album, “Believe,” landed in June and immediately perched atop the Billboard 200.

His legion of Twitter followers (33 million-plus) outnumbers the populations of North Carolina, South Carolina and Georgia – combined.

Until he turned 18 last March, though, he was just a kid. And adults typically don’t like kid things. Or if they do, they won’t admit to it.

“I think Justin Bieber is becoming everyone’s secret indulgence,” says John Reynolds, program director for Kiss 95.1. “He still appeals to the young audience, but he’s broadened his base by teaming up with artists like (rappers) Big Sean, Nicki Minaj and Ludacris on this new album. That’s helped make his music a little bit more mature, and that opens the door to an older demo.”

Of course, for some older fans, the indulgence is not-so-secret.

Gina Sirhan, 41, proudly displays a large poster of Bieber tousling his own hair over her workstation at Invisible Fence of the Carolinas. On her desk are Bieber notepads, pens and a Valentine’s Day card adorned with the Biebs’ likeness.

“He’s just a genuine, really good-hearted person,” she says, noting that he has always put a premium on his fans and on philanthropy. (The singer is known for visiting sick kids at hospitals during tour stops; he asked Twitter followers to celebrate his 18th birthday by donating to charity:water, a nonprofit that brings drinking water to people in developing countries.)

As Sirhan is in the middle of explaining her fandom, a co-worker walks by and ribs her.

“It’s not a freaky fetish! It’s not! It’s not!” she yells.

Then another one piles on, and she shoots back: “I’m not too old for that, stop it! He’s not underage! He’s 18! And I’m not touching him!” Her eyes roll. Her co-worker’s do, too.

Sirhan’s office is good-natured about its teasing, though – co-workers pitched in and got her a Bieber-themed cake for her last birthday.

What’s that, you say? A 40-something woman with a Bieber-themed birthday cake? Gotta be a first, right? Wrong.

Kristalyn Thornock, a Concord stay-at-home mother of three, was thrown a Bieber-themed 40th birthday party by her two sisters.

“I seriously had the same cake as my 7-year-old,” says Thornock, who gave her daughter tickets to Tuesday’s concert for Christmas and says – with no hesitation – “I’m just as excited, if not more than she is, for it.”

Meg Santanna, 33, who is going to the show with her 10-year-old stepdaughter, Grace, told the Observer the same thing, but added, with a laugh: “Part of me will be breathing a sigh of relief if you don’t (quote me).”

Guilty pleasure

Yup. Someday, the stigma attached to being a grown-up fan of Bieber might go away, but for now Kimberly Schroeder is just glad her co-worker Sheri came through with two kids she can “adopt” for the night.

“No one thinks twice about going to a Madonna concert, or to a Jack Johnson concert, or Dave Matthews. But when it comes to these teeny-bopper ones (it’s a different story). … It’s a shame that I wouldn’t have enough guts to do it by myself.”

Schroeder pauses, then – with only the slightest hint of defensiveness: “I think there are a lot of mothers that actually listen to their kids’ (music) and actually like him, but are afraid to admit it. And at the gym, they play his songs, and you can see the guys actually singing along when they’re running to it, too. It is kind of funny.”