Joy Ahrens grew up in Southern California, where cars are king. She got her learner's permit when she was 15 and eagerly waited for her next birthday and her chance to take her driving test. But not long after she started learning to drive, Ahrens' parents divorced and her mother was left to raise six children on very little money.

"We were on welfare, and my mom couldn't afford the extra insurance on the car," Ahrens, now 25, said.

While her peers could get jobs to pay for things like car insurance, Ahrens could not. Her job was taking care of her younger siblings. The strain was too much for her, and when she was 18, she left home to live with her grandmother in Washington. She got jobs at a retirement home, the Dollar Tree, a coffee shop, the Boys and Girls Club, at summer camps and as a nanny. But she never could save up enough money to get a car.

Like Ahrens, young adults across the country are postponing what used to be a rite of passage--getting a drivers' license at 16. Researchers blame some of the shift on economics. Fewer families can afford to buy their teens their own car and pay for extra insurance and gas. But social media also plays a role. Today's young adults don't depend on face-to-face time with friends for their social interactions, which makes driving less important.

According to a new study conducted by Frontier Group, an environmental think tank, today's young adults are walking, biking, and taking public transportation more often. Between 2001 and 2009, 16-34 year olds drove 23 percent fewer miles, took 24 percent more bicycle trips, walked 26 percent more times, and took public transit 40 percent more times per capita, according to the study.

And between 2000 and 2010, the number of young adults between 14 and 34 years old who do not have a license dropped by 3 percent.

Cedarville University, surrounded by fields in the tiny town of Cedarville, Ohio, is one campus that has witnessed this trend. Mark Weinstein, executive director of public relations, told World on Campus the school has seen a 3 percent decrease in the number of students bringing cars to campus since 2010.

When Ahrens enrolled at Bellevue College just east of Seattle, Wash., her friends were reluctant to let her use their cars or help her learn to drive. Some of them lacked the necessary insurance, or had not had a license long enough, to legally train her.

She frequently asked her friends how they got their licenses or their cars. They responded with surprise that she did not have either, and told her that their parents had played a big role in helping them get their transportation independence. But Ahrens' parents weren't around to help, and she had to settle for riding a bicycle. At one point, Ahrens was getting up at 5 a.m. to ride her bike in often icy, rainy weather to open the coffee shop where she worked. She once lost a nanny position because the parents found out she could not drive.

Last fall, she finally got her license after a friend taught her to drive.

"It just took me a long time to find somebody who would be willing to let me use their car, and who had insurance, and who had had their license for five years or longer," she said.

But even though she had her license, Ahrens still didn't make enough money to pay for gas, insurance or maintenance on her own car.

The high cost of owning a car is the "most significant reason for reduced driving," according to a 2010 study commissioned by Zipcar, a popular car-sharing network, and conducted by the global market research firm, KRC Research. The survey found that 80 percent of Millennials questioned said it could be difficult to own a car in the current economy.

But social media also plays a roll in young adults' delayed driving habits. According to the Zipcar study, more than half of Millennials said they preferred online face time to driving for a face-to-face visit with friends.

Andrew Crites, a 27-year-old instructor for the Oregon Driver Education Center, agrees.

"I think it has to do with the ability to stay connected with your friends without leaving your house," he said.

As a teen, Crites had to drive somewhere to see his friends. Now, teens and young adults can get on Twitter or entertain themselves with video games. It is easier, faster and free to interact with friends online, and even young people who can drive may choose Skype, Gmail and Facebook chat over getting in a vehicle.

Crites has been a driving instructor for six years, and said that total numbers at the driving school have remained steady. But the average ages of students taking the class have shifted. Six years ago, Crites taught mostly 15 and 16 year olds. Today, most of his youngest students are 16 and 17 years old.

"It's not that there are fewer," he said. "It's just that they are waiting longer."

After Ahrens got her license, she found a better-paying job as a live-in nanny in Mercer Island in Washington for three children. She eventually saved enough to buy her first car--a dark green, 1997 Geo Prizm. She bought it three months ago on Craigslist for $1,700. It still runs well.

Getting both a decent job and a car seemed like an incredible blessing after all the years of waiting and working, she said: "Through it all, I learned to work really hard and trust God."