The company is far from the first niche gym — Curves International Inc., which targets women 35 and older, debuted in 1992 and has become the largest fitness franchise in the world, with 10,000 locations in 42 countries. Curves is based in Woodway, Texas. There are about 1.5 Curves for every two McDonald's in the United States.

Although Overtime was founded exclusively for teenagers, in January it opened to women from 6 a.m. to 2 p.m. It is opening to men this week and with expanded overall hours.

Like other gyms, Overtime caters to customers who can afford to pay for a place to exercise. But experts note that low-income teens are at the highest risk of obesity, diabetes, asthma and other health problems.

"Fitness is more akin to a public good, especially for kids," said Ann Cotten, director of the Schaefer Center for Public Policy at the University of Baltimore. "I worry that the kids that get access to this gym are the same ones on private soccer leagues."

The company is considering asking Mountain View-based Google Inc. and other local businesses to fund memberships for lower-income teens. It says it hopes to reduce teens' monthly fee as it gets more revenue from adults. Currently, day passes are $10, or five for $40.

Ximena Urrutia-Rojas, an associate professor of social and behavioral sciences at the University of North Texas, said emphasizing teen health is good but is no substitute for an active lifestyle that involves the whole family.

"Even teenagers who say they want to separate from parents feel motivated when parents or other adults initiate the activity," she said.

Sarah Barlow, associated professor of pediatrics at Saint Louis University, praised Overtime's novel approach.

"Even for adults, the treadmill and stationary bike don't sustain interest over time," Barlow said. "I like the idea of taking video games, which are so successful at engaging kids, and modifying them to get kids engaged in physical activity — now that's fun."