Teens have been hanging out online for 20 years, but in 2017 they're doing it on group video chat apps, in a way that feels like the real thing, not just a poor substitute. Ranging in age from adolescents to their early 20s—the group loosely defined as "Generation Z" —these young people are leaving the apps open, in order to hang out casually with peers in a trend some call "live chilling."

This phenomenon is made possible by the sudden ubiquity of video chat, in messaging apps such as Kik and Facebook Messenger , as well as stand-alone apps including Houseparty, Fam , Tribe , Airtime and ooVoo....

These apps make sense now in part because more teens than ever have access to smartphones. In 2015, the Pew Research Center reported 73% of U.S. teens have access to a smartphone, and that figure is growing. Those teens are checking their phones on average more than 80 times a day, according to Deloitte .

It isn't just that teens have phones, and that the infrastructure required to handle multiple simultaneous video streams is more accessible to developers than ever. It is also that teens aren't getting out to socialize in real life like they once did. One in three teens told Pew that they hang out with friends outside of school less often than "every few days."

"To me that's where this story begins," says Ted Livingston , founder and chief executive of Kik, a messaging app with 300 million registered users that is especially popular with American teenagers. It is hard for older generations to understand how young people cope with this lack of physical hangtime, he says. "The answer is they're hanging out with their friends on their phone."

Mr. Livingston says that when teens leave Kik's group video chat open for hours, it is a sort of passive window into friends' homes and lives. This behavior isn't so different from the way Generation X would call friends after school, and millennials used AOL Instant Messenger and, later, text messaging to keep up with friends, says Ryan Hoover, founder of startup tracking service Product Hunt.

To a large extent, all these technologies have been an adaptation to teens' inability to access one another in person, says Jan Odiaga, assistant professor at Rush University College of Nursing in Chicago, who studies how technology influences activity levels in young people. The situation is worse than ever because of packed schedules, helicopter parenting and the decline of walkable neighborhoods.

The phenomenon is exemplified by a recent case that made national headlines : When parents in Maryland allowed their children to walk home from school on their own, police and child protective services got involved. Many parents feel the frustration that social pressures prevent them from allowing their children the same freedom of movement that they themselves had as teens.

The net effect, says Ms. Odiaga, is that teens are spending more time indoors, and are less active, than ever. Studies examining time use since 1965 show a significant decline in active time accelerating in the mid-1990s , to the point that young people today are sedentary for more than 10 hours a day, says Ms. Odiaga. We're at the point now, she adds, that the technology is also driving the trend.