Retailers need to create new shopping experiences for millennials.

Millennials—born between 1980 and 2000—are both the 20th century’s last generation and its first truly digital one. This old century/new technology dichotomy gives pause to marketers attempting to understand and connect with this key demographic.

At the same time many traditional apparel retailers are struggling.

Macy's reported a disappointing outlook as executives say consumers are spending their dollars elsewhere. Michael Kors' inventories are rising amid criticisms that the brand isn't cool anymore.

Teen retailers like Abercrombie & Fitch, Aeropostale, and American Eagle are downsizing and changing their strategies. And apparel stores like C. Wonder and Wet Seal have shut down locations this year. 

What's the biggest problem for these companies? 

Millennials, according to recent data from consumer information company NPD Group. 

"The fashion industry has undergone one of the most dramatic makeovers in recent history – no doubt influenced by the Millennial consumer," said Marshal Cohen, chief industry analyst at NPD. 

While previous generations dressed up for work and play, millennials have a tendency to wear brands like Nike, Under Armour, and Lululemon everywhere, according to NPD. 

"There is an underlying sense of rebellion that comes through in today’s fashion," Cohen said. 

Cohen says that traditional retailers will have to evolve to newly casual consumers. 

When stores close - and many are - malls make cutbacks, which leads to a sense of deterioration.

“It’s a domino effect,” said Tatelbaum. “And when the consumer feels the mall is dilapidated, that’s one more reason they’re not going.”

Besides the shift away from mall shopping, there’s the normal, fickle nature of fashion that teen retailers have to contend with.

“It used to be you wanted to fit into a mold and everyone looks the same,” said Sidney Morgan-Petro, retail editor at trend forecasting and analytics firm WGSN, discussing teen shoppers. “But now that consumer is thinking about how they can stand out.”

Teens have moved away from the logos, the American collegiate and the varsity appeal that used to dominate at retailers like Aeropostale. Right now, for example, the 90s are back, which means a more pared back and basic look, like the slip dress.

“More variety and diversity is what the junior consumer really wants,” said Morgan-Petro. And, she says, they’re finding it at fast-fashion retailers like H&M, Zara and Forever 21.

Fashion retail, like hemlines, is always changing. Even as companies try to adapt, Cohen says we should expect more of them to run into trouble.

“Retail is never done when it comes to bankruptcies and transitions,” he said. “It doesn’t mean they’re going out of business, just regrouping. You never know who is going to be next.”