Age is just a number, especially when you're 47 going on 12.

Nora Whalen and her identical twin sister, Ruth Whalen Crockett, were born on Feb. 29, 1976. That would make them now 48, but because Leap Day comes only once every four years, they've only had 12th true birthdays — including the one they celebrated Thursday night in north Minneapolis at a throwback party with several dozen family members and friends.

To celebrate the twins turning 12 again, well-wishers packed La Doña Cervecería for a evening that culminated with circa 1988 karaoke to relive the first time the sisters were 12. Belinda Carlisle, George Michael and Mr. Big graced the playlist, and this mostly Gen X crowd, along with their kids, belted out tributes to their favorite leaplings.

What's it like being born on a day that is as rare as the Olympics or a presidential election?

"It's the year I realize the people around me are math-challenged," Nora said, noting that they can't seem to divide or multiply easily in their heads by four. But it's special, too. "Every four years I hear from people I haven't heard from in four years."

This Leap Day birthday was even more special because Nora got to party with her twin, something they haven't done since they were 16 (or 4, depending how you're counting). Ruth, who lives in Lowell, Mass., flew in to Minneapolis Thursday to join her sister for the festivities.

The crowd sang happy birthday to the sisters. And the twins, clad in matching Dickies pinstripe overalls, returned the favor with a gutsy rendering of "One Moment in Time," which Whitney Houston crooned at the 1988 Summer Olympics in Seoul.

The world is home to about 5 million people who were born on Leap Day. The odds of having a Feb. 29 birthday are already slim, but they're even rarer because obstetricians are scheduling fewer cesarean deliveries on that date, according to the polling aggregation site FiveThirtyEight. Many parents don't want to deliver on Leap Day, afraid of relegating their children to birthdays that only return every four years.

But Ruth said she wouldn't want to have been born on any other day.

"I think it's a great day to have a baby," Ruth said. "People always remember your birthday. My second-grade teacher, Ms. Ross, always contacts me on my birthday."

The story of the Whalen sisters' births in Madison, Wis., was momentous in more ways than one. Her mom had no idea that she had two babies inside her.

"First she delivered me, and the doctor told the intern, 'OK, clean things up.' The intern says, 'Hey, there's another one in here,' " Nora says. "It was shock upon shock on Leap Day."

Growing up in Monona, Wis., the sisters made sure to keep the surprises coming. They pranked the adults in their lives, like by switching classrooms to see if their teachers would notice. The Whalen girls, known in town as the Leap Twins, were nearly dead ringers for each other.

"We're mirror-image twins," Ruth explains. "She's right-handed. I'm left-handed. We each have a mole but it's on the opposite side."

As they grew older, they tried to lead different lives — playing different sports or instruments and mixing it up with different friends. They tried to branch off in life, Ruth to Beloit College and Nora to St. Olaf. But they ended up with the same exact G.P.A. and eventually in the same profession: teaching.

On several occasions, they've even pulled practical jokes on their own students, with one sister substituting for the other in the front of the classroom. Sometimes it would end with the real sister jumping out from under a desk.

Nora said it's been fun to turn 12, the age of her twin boys, Isaac and Oliver. She and her husband, Ben Goldfarb, also have a 15-year-old daughter, Emilia, who on Thursday at South High School was greeted by friends from as far back as elementary school wishing her mom a happy birthday.

The last time Nora and her husband hosted their Leap Day bash, it was a smaller gathering right before the COVID-19 shutdown in 2020. Their friend, Kate Hopper, said that occasion four years ago was the last gasp before everyone became hermits.

"We had no idea what was coming: the shutdown, how hard it was on the kids, George Floyd," she said. On Thursday, she said, "it almost feels like it's a mark. We made it through, we've all changed, but now we're able to come together and celebrate."

Every Leap Day, Nora says, she takes stock of the past and envisions her next four years. It also infuses some levity to what could be a dreaded birthday. Over the years, her friends will say, "Well, Nora's only 10," or "Nora's only 11." Hearing those words makes her approach aging with a sunnier perspective. She looks forward to being 64, so she can celebrate with a proper Sweet 16.

"We get to be the ages we feel or want to be," she said. "When I turn 21, I'll be 84. Like, how fun will that be?"

It'll be even more fun if her twin is by her side, singing and partying with her like it's 1997.