Sarah Mirmelli is ready to face the music.

Before heading to the Coachella Valley Music and Arts Festival in Indio, Calif., this weekend, the 33-year-old divorced mother of two blew $20,000 in preparation. Her clothing alone for the weekend — a “sick” outfit featuring a vintage Guns N’ Roses T-shirt, YSL army jacket, denim cutoffs, corset, thigh-high leather boots — was $8,000. She spent thousands more on Botox, lip injections, boxing classes and a personal trainer who helped her drop 10 pounds in advance of the three-day-long concert in the California desert. In the month leading up to it, she even gave up alcohol and went vegan to get in shape for the debauchery to come.

‘Getting ready for it [Coachella] doesn’t have to be a full-time job, but I’m making it one.’ - Sarah Mirmelli

“Everything’s about Coachella,” says Mirmelli, who started going to the desert festival after needing to cut loose following her divorce six years ago. This year is her fifth time attending.

“[Getting ready for it] doesn’t have to be a full-time job,” she says. “But I’m making it one.”

The annual festival has long been a place for teens and 20-somethings to frolic and Instagram while inebriated and scantily clad in a 78-acre polo field, while also taking in a cacophony of pop, rock, electronic and rap music.

Nubile young celebs like model/actress Emily Ratajkowski, 24, and model/DJ Chelsea Leyland, 25, are expected this year — and are sure to post plenty of sexy selfies in boho-chic garb. But, increasingly — and thanks to a growing number of nostalgia acts on the lineup, like this year’s headliners Guns N’ Roses and LCD Soundsystem — the fest has also become a mecca for older women. They’re indie-rock gals looking to get their grooves back after bad breakups, and uptown wives clinging to their last vestiges of youth. They spring for the $899 VIP passes so they’ll have access to fancier porta-potties than the young plebes only shelling out $399 for regular tickets — and they spend even more to look their best on the big weekend.

“You can go like a peasant, too, but it’s going to suck,” says Mirmelli, who splits her time between Miami and Midtown and works as an associate publisher for Haute Living (when she’s not prepping for “ ’Chella”). “The goal is looking blog-ready.”

New York doctors who specialize in Fountain-of-Youth procedures say they’ve noticed an uptick in ladies looking for nips, tucks and tightenings before heading to Coachella.

“About two months ago, when Guns N’ Roses mentioned they would [play], more women in their 40s started coming in,” says Dr. Norman Rowe, an Upper East Side plastic surgeon whose main focus is fat-reduction procedures. “They’re reliving their teen years.”

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He recalls one patient in her mid-30s who pleaded with him, saying, “I want to wear this crop top like I did when I saw Guns N’ Roses back in the ’90s.”

But music is only part of the festival’s draw. There are pool parties and younger guys to flirt with. And it’s a nice opportunity to rent an elaborate vacation home; Mirmelli and a gaggle of girlfriends are sharing a “sick house” in tony La Quinta, complete with a makeup artist-in-residence, a pool and a movie room. Last year they even traveled in a private jet.

“It’s fun for me to feel cool,” Mirmelli says. “You get down with people you normally wouldn’t speak to. They wouldn’t normally have access to you.”

Mirmelli’s friend and fellow Coachella-goer, Brooklynite Annette Cooper, 36, is so intent on looking cool that she hires a personal stylist, Brandon Fogel, to source the right clothes.

“He styled me last year; he knows my style. I’m very much a bohemian, flowery-girl type,” says Cooper, a mom of three who first attended the festival last year — wearing designer duds by Zimmermann and Balmain — after her 15-year marriage ended.

“I was 35. I said, ‘I have to do it.’ I always wanted to do it.”

Designer labels aside, she says there are practical considerations to keep in mind.

“I’m normally a girl with heels, but you really have to have a comfortable shoe, because it’s all about the music,” she says, but then quickly admits that it’s not really all about the music.

“The draw is the people that go,” she says. “It’s amazing — we met oncologists, real estate developers, bankers. I was amazed.”

Aside from men with status jobs, there are, of course, plenty of pretty young things in the desert. That competition led Erin Crowley, 33, to phone her plastic surgeon right after buying her tickets earlier this month.

“I called and said, ‘What can I do that’s going to show right now? I need something strong.’ ”

The single mom paid $8,000 to have fat removed from her stomach and inner thighs earlier this week before she heads to Coachella next weekend (the festival takes place, with the same lineup, over two weekends). She also dyed her hair, got fake eyelashes and had her lips plumped by her plastic surgeon, Dr. Rowe.

“I need to do my best to prepare for the outfits that I’m wearing,” says Crowley, who works as a hairdresser and lives in Putnam County. “I’m kind of wearing short shorts and crop tops.”

And, she notes, the competition is tough.

“It’s all young girls,” laments Crowley, who’s hitting the fest with her 23-year-old baby sister, who has a “perfect little body.”

“[She] is a selfie queen,” Crowley says. “I want to look my best for the photographs, and you can’t hide anything in natural light.”

But Mirmelli says she’s not intimidated by millennial music lovers.

“I have the confidence that they don’t have,” she says. “They’re younger and they don’t have careers. They act all entitled, but they don’t have anything to back it up.”

Christina R., 39, a married mother of two, is headed to Coachella with some girlfriends after spending about $5,000 to have the skin on her stomach and the backs of her legs tightened.

“I’m feeling very confident — my whole abdomen is tight,” says the Upper East Sider, who works at an art gallery and has a $10,000 clothing budget for the weekend, mostly devoted to YSL crop tops, bikinis and jean shorts “cut off around the tush.”

“Hopefully I’m not too old,” says Christina, who declined to give her last name for professional reasons. “I’m so young at heart — when I get away from my husband and the children, I don’t really feel old.”

And while her husband doesn’t really understand her festival-frolicking needs, he’s supportive.

“He just wants me to be happy,” she says.

A woman is never “too old,” according to Patricia Field, the celebrity stylist who did the costumes on “Sex and the City” and also works on the current TV Land show “Younger,” about a 40-year-old woman trying to pass for 26. Field went to Coachella for the first time last year — at the age of 73.

“In life, you have to live it, you can’t go by notions of what you do at this or that age,” she says. “You have to live your life by how you feel.”

But some women admit, after going to Coachella, that maybe they’re too old for it.

“I’m a mom, I’m not walking around in a flower crown,” says Jamie Hess, 35, who works in PR and lives on the Upper West Side. She went last year when she was five months pregnant, and is going this year, but she’s self-aware about what she sees as age-appropriate.

“When I’d go to Madison Square Garden as a teen, and I’d see the mom from Jersey trying to relive her youth in her bangs and hairspray, it’s like, ‘No, honey!’ ” she says. “No one wants to be seen as the geezer who’s trying too hard.”

She says the festival has become a common event in her social circle. “I was literally at [a party] two weeks ago, and an hors d’oeuvres plate came around and someone around my age said, ‘Oh no, Coachella is coming up.’ ”

Patricia, a 42-year-old from the Upper East Side who declined to give her last name for privacy reasons, isn’t one of the women watching her waistline. She went to Coachella last year and has no plans to go back.

“It’s really young,” she says, complaining of the music being too loud and her hotel being a 24-hour party. And then there were the fashions.

“I felt like all the young teen girls were wearing high-waisted jean shorts with their ass hanging out. But they’re young, they can do that,” she says. “I’m too old for it. If I was younger, it would be a lot cooler.”