Having our cake and eating it, too!

A ponytailed girl, gussied up in a frilly fuchsia dress, bicycled through SoHo Wednesday toward her birthday party, streamers flying from her handlebars. She tapped the kickstand and skipped toward a room filled with balloons, cupcakes, ice-cream cones, lollipops and girls dipped in glittery pink party dresses. It was a birthday worthy of a 7-year-old princess.

Or, if you’re Betsey Johnson, a 68-year-old fashion designer.

“Kids just have more fun on their birthdays,” Johnson tells The Post, explaining her childish shindig. That same evening, New York Rep. Charles Rangel threw himself a birthday party fit for little Eloise at The Plaza. Despite pending ethics charges and the 80 candles on his cake, the Harlem Democrat celebrated with a singalong to his BFFs (Dionne Warwick performed “That’s What Friends Are For”), piles of greasy onion rings and playground potty talk (“This damn sure ain’t no funeral, is it?” Rangel kidded). Like Johnson and Rangel, many adult New Yorkers have decided their birthdays are an excuse to go big — go kid or go home.

SEE THE PHOTOS: GROWN-UP BIRTHDAYS

Bored by tedious group dinners, competing with the city’s endless entertainment temptations, feeling overworked and underappreciated — or simply enamored of their own awesomeness — these Peter Pans refuse to let their birthdays grow up.

Johnson says her birthday fetes are “never formal, always fun, over the top, total cornball. The whole nine yards. Games, hats, decorations — real crazy fun times.”

Comedian and “30 Rock” star Tracy Morgan says the thing he wants most for his 42nd birthday this November is a huge Superman cake, identical to the one his mom baked for his sixth birthday. The only difference from his childhood birthday plans?

“You know I gotta have someone jumping out of the cake,” he tells The Post.

Cake jumping or no, re-creating beloved childhood memories is a growing theme for grown-up birthdays citywide.

Event planner Matt Toubin, owner of Shine Events, says about 10 percent of his clients are New York adults planning kid parties for themselves. (The numbers are likely much higher for self-planned soirees.) Inspired by children’s parties, these grown-ups demand water-balloon races, roller skating, Pac-Man arcade games, cotton-candy machines, snow cones and “Pin the Tail on the Donkey” — all of which Toubin bemusedly provides. He describes one juvenile birthday blowout he planned for a wealthy “finance guy” in Rye, NY, last May. The birthday boy, turning 45, begged for a big-top theme complete with carnival games (with goldfish and stuffed toy prizes), circus tents, face painters, stilt walkers and cheese fries.

“New York is such a pressure-filled city, these people who work in finance or legal professions put in thousands and thousands of hours — they want to celebrate in a bigger way,” he says.

“These parties let them revisit their past, to a time when everything was fun and they didn’t have to worry.”

Union Square denizen Drew Sanocki is engaged to be married, hosts sophisticated wine-and-cheese gatherings and runs a high-end home design retail site called Design Public.

But each December, when his big day rolls around, he’s like a kid in the birthday store.

Last winter he wowed his pals with an R2-D2 fondant cake, a Darth Vader piñata, an accordion player churning out “Star Wars” tunes, lightsaber party favors and “May the force be with Drew” stickers.

“I was going to have 20 Imperial Stormtroopers come in and maybe arrest somebody, but they had their Christmas party the same night as my birthday party,” says the seriously bummed Sanocki.

Well, there’s always the next b-day. Which will be his 39th.

“Childlike parties are much better because they’re unexpected,” he says.

“A few years ago, when ‘Sex and the City’ was at its peak, your birthday was all about finding the hippest new club and getting all your friends on the guest list — but those parties sucked.”

Sanocki says his guests preferred his immature celebration: “Even the most stuck-up New York friend was letting their hair down and hitting the Darth Vader piñata.” (There are limits: Sanocki asked all 25 of his female guests to dress up as Princess Leia. “Only two did,” he admits. “One was my fiancée.”)

While the rest of the country is busy planning parties for their own children, most New Yorkers are kid-free. (Two-thirds of New York metro households don’t include children, according to the latest US Census data). Without real wee ones to celebrate, they get to act their shoe size — not their age — on their birthdays for as long as they like.

Another reason big-kid birthdays are bigger business in the Big Apple than other cities? New Yorkers notoriously suffer from social-calendar ADD.

“When you’re in a place like New York, there’s just so much going on at any given time, you need to give people a good reason to come out,” says Pete Marinucci, an art historian living in Astoria.

He coaxed pals to his 30th birthday last June with a skee-ball party at the Full Circle Bar in Williamsburg. (The bar boasts three lanes and an official skee league.) He promised wooden balls and high rollers, prize tickets and piñatas. And the skeè de résistance: a birthday cake in the shape of a skee scoreboard.

“Birthdays are the one day of the year when you’re special and you’re the center of attention,” he says. “When you’re a working professional, one day blends into the next and it’s a constant slog, but your birthday is the time when you can really break out of that.”

But isn’t 30 a bit old for a skee-ball b-day?

“Definitely not,” Marinucci insists. “You’re at your peak of skee-ball performance at 30.”

Ashley Albert knows all about partying like a kid rock star. She’s the founder of the wildly popular “kindie band” the Jimmies and the voice of Nickelodeon network promos.

For her 37th, last April, she hosted a murder-mystery birthday and asked friends to dress up as their assigned characters. Every year, she also “makes” her two best friends sleep over at her apartment in Gramercy Park.

“On my birthday, I get the leeway to be as ridiculous and childish as I want to be and everybody is forced to play along,” she says with a giggle. “In New York especially, we’re all chugging away at our goals, but for one day, you get to say, ‘Instead, I’m going to eat a cupcake.’ ”

BIRTHDAY BOOTH

The new kiddie party zone for adults

One of the best new venues for birthday celebrations is at the Hester Street Fair on the Lower East Side, which rents birthday booths Saturdays and Sundays for $100 per day and lets birthday boys and girls kick it, kid style. The fair played host to a babyish 42nd birthday bash for Ivan Wicksteed, vice president of global marketing at Converse, last April. Wicksteed’s wife, Annie Yun, orchestrated a vintage photo booth, a giant happy birthday poster and bouquets of balloons printed with Ivan’s face and the message “Happy Birthday Ivanator.” Yun says the silly spectacle drew neighborhood kids, street-fair shoppers and even local cops. “It was one of those New York moments — it was nice to share it with other people,” she says.

Last month, George Yzquierdo, who owns the Treasures & Pleasures antique jewelry shop, hosted his 55th at the fair in the style of a childlike tea party — complete with food tickets, balloons and punch.

“I’ve had fabulous birthday parties in fabulous locations — but this one was about as fun as it gets,” he says.

It’s a sentiment echoed by Sway Calloway, a “30-something” MTV News correspondent who partied young at the birthday booth two weeks ago with fruit Popsicles, mini cupcakes and a deejay.

“I’ve thrown birthday parties in St. Thomas . . . in Miami — this time, I wanted to do something to differentiate myself,” he says. “People will be thinking, ‘Sway did a party at a street fair — what does that mean exactly?’ ” Maybe that, like the rest of us, he doesn’t want to grow up — he’s a big birthday kid.