In the age of the smartphone, tweens and teens prefer online video over most other media.

U.S. kids ages 8 to 18 have more than doubled the time they spend watching online videos in the past four years, to nearly an hour every day. For tweens, who reported spending 56 minutes a day watching videos mostly on YouTube, it is now what they enjoy doing most — more than watching TV, listening to music, using social media, playing video games or mobile games, and reading, according to a new survey released this week by Common Sense Media. For teens, who spent 59 minutes a day watching online videos, it ranked second, after listening to music.

More than twice as many young people watched online videos every day in 2019 than they did in 2015, the last time San Francisco-based nonprofit released a study about youth media use. Among 8- to 12-year-olds, 56% reported doing so every day, compared with 24% in 2015. Among 13- to 18-year-olds, 69% said they watched online videos every day, compared with 34% in 2015.

That shift — partly attributable to a majority of kids getting phones by the time they are 11 or 12 — has come at the expense of television. According to the study, only 24% of teens’ TV viewing was on a television set, down drastically from 2015, when that number was 48%. That amounts to a drop of about 25 minutes a day among tweens and 24 minutes a day among teens.

“The study shows worrisome indicators as our most vulnerable population — our kids — are spending a lot of time on unregulated, unrated platforms that deliver content that can be inappropriate or even dangerous,” said James P. Steyer, founder and CEO of Common Sense, in a statement.

YouTube has been criticized for allowing extremist and hateful content on its platform, its algorithms that recommend supposedly related videos to users, and more. (Also, last month the Google-owned company agreed to pay $170 million to settle charges it collected children’s data without parental consent.)

Studies that look at screen time “unfortunately don’t tell us all that we need,” said Tom Robinson, professor of pediatrics and health at Stanford University School of Medicine. “People can go to college on YouTube,” he said. “Or they can be watching porn.”

The study did point to possibly positive use of online-viewing time: 65% of tweens and 63% of teens said they watched videos about how to make, build, or do something they are interested in.

Still, Robinson added: “Right now, it’s the Wild West out there. It’s driven by a business model that is all about getting people’s attention for their advertising, without really being to evaluate what the impacts are.”

YouTube dominates as the online video platform of choice among kids, with 53% of tweens and 59% of teens saying they watch it the most. The next closest service is Netflix: 27% of tweens and 31% of teens said they watch it most.

Total hours of screen time for tweens was 4:44, up from 4:36 in 2015. Teens’ screen time had a bigger increase to 7:22 from 6:40 four years ago.

The study surveyed 1,677 U.S. 8- to 18-year-olds about their use of and relationship with media from March 11 to April 3, 2019. The margin of error was plus or minus 2.8 percentage points.