When Abby Ellstrom and her friends at East High School in Denver crave classic chicken nuggets for lunch, the sophomores swipe the Uber icon on one of their smartphones and ride the nine blocks to Chick-fil-A. When they are done, they click Uber again for pickup—and make it back in time for their next class. Each girl gives a few dollars to the one who charged her parents’ account.

Abby, age 15, says she has taken Uber cars to school sports events, the mall, the nail salon and a Big Gigantic concert at Red Rocks Amphitheatre. “We use it all the time,” she says. “We’re a lot more mobile for kids without licenses. It gives us a lot more independence.”

Uber Technologies Inc., with more cars on the road than its rival rideshare car services including Lyft Inc., Sidecar Technologies Inc., Shuddle Inc. and Boost has become a routine form of transportation in some families. Many teens book travel without adults, achieving new levels of autonomy.

Uber, Lyft and Sidecar have explicit policies that forbid children under 18 from establishing an account and riding unaccompanied. Uber says, “Users agree to our terms and conditions when they download the app,” says Uber spokeswoman Kristin Carvell. “We expect them to adhere to them. If we were to discover that someone under 18 had set up an Uber account, the account would be deactivated.” When asked about additional consequences of someone violating the policy, Ms. Carvell said she had nothing further to add.

Families are embracing Uber cars despite the policy. They are also using the service as regulators in several states ramp up scrutiny of rideshare companies’ business practices, including questioning their background checks of drivers.