They’re narcissistic. Apathetic. Pampered. And addicted to their four-inch screens.

If you believe the conventional wisdom about the millennial generation — those 16 to 34 years of age, by most calculations — you’ve got considerable reason to worry about the future of the U.S. economy. Millennials show far less interest in buying cars, homes and other big-ticket items than their parents did at the same age, which has generated an intense effort among companies that produce those things to crack the code of these crazy kids and figure out how to sell them stuff.

View photos Yahoo! Finance More

But the millennials may not be as mystifying as an army of sociologists makes them out to be. “Every generation eventually sheds their most extreme characteristics,” says Jason Dorsey of the Center for Generational Kinetics, a consulting firm in Austin, Texas. “What is different about millennials is delayed adulthood. They’re entering into many adult decisions later than ever before.” And the reason may be fairly straightforward: They don’t have much money. Not yet, anyway.

[See related: Why More Families Are Shunning Cars]

One of the biggest mysteries of millennials is why they seem to have little interest in cars, which have been an irresistible source of freedom and mobility for young people since the interstate highway system opened the whole country to Chevys and Mercurys in the 1950s. Yet millennials seem to scoff at the open road. The percentage of 16-to-24-year-olds with a driver’s license has dropped sharply since 1997, and is now below 70% for the first time since 1963. “Millennials are demonstrating significantly different lifestyle and transportation preferences than older generations,” declared a recent report by the U.S. Public Interest Group. Overall, it concluded, “the driving boom is over.”

Smartphone: The new starter car?

One common theory is that the smartphone is the new starter car, with social networks providing new kinds of freedoms for young people, and new ways of connecting with friends. Yet millennials, as a whole, are also buying homes later than prior generations, having children later and delaying their careers. It’s as if America’s youth are rejecting social conventions that generations have held in common for decades.

At a recent panel discussion on millennials sponsored by Ford Motor Co. (F) in New York City, a fairly mundane explanation surfaced: This generation simply faces a far tougher economy than their elders tend to realize, which has made it much harder to reach traditional life and career milestones. “We attract a ton of millennials,” said David Rabkin of American Express (AXP). “But we aren’t able to approve them for a lot of products that we have. Young people who once graduated from school and would go into a traditional corporate job now might move back in with their parents.”

There’s plenty of evidence that younger workers may face the most difficult economy since the Great Depression. The national unemployment rate is 7.5%, but it’s 16.1% for 16-to-24-year-olds. Many baby boomers are working longer, to rebuild wealth lost during the recent recession, which has pushed the retirement age to the highest level in more than two decades. That has reduced turnover in the labor force, further limiting openings for younger workers in an already challenging job market.

View photos Yahoo! Finance More

Many young people have done what they’ve been told to do and gone to college, since education remains an important pathway to success. But many are graduating with heavy student-debt burdens and finding they can’t get jobs that pay enough to make the hefty payments on those loans. A recent study by the Federal Reserve Bank of New York found that high student-debt loads may be one of the biggest reasons young people buy fewer cars, homes and other appurtenances of modern life.

There are a few other reasons younger people may not be embracing cars the way their freewheeling parents did. Insurance premiums for young drivers can be exorbitant these days. With more focus on safety, a lot of parents don’t want their teenage kids behind the wheel and they’re quite happy to delay "driving day." And more Americans of all ages are moving closer to cities, which cuts down on the need for a car.

Story continues