Do you have a teenager at home? If so, he or she probably isn't exercising enough.

One of the largest studies of its kind shows just how sluggish American children become once they hit the teen years: While 90 percent of 9-year-olds get a couple of hours of exercise most days, fewer than 3 percent of 15-year-olds do.

What's more, the study suggests that fewer than a third of teens that age get even the minimum recommended by the government -- an hour of moderate-to-vigorous exercise, like cycling, brisk walking, swimming or jogging.

The sharp drop raises concerns about inactivity continuing into adulthood, which could endanger kids' health throughout their lives, the study authors said.

"People don't recognize this as the crisis that it is," said lead author Dr. Philip Nader, a pediatrician and professor emeritus at the University of California at San Diego.

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The study, appearing in today's Journal of the American Medical Association, tracked about 1,000 U.S. children, from 2000 until 2006.

Special gadgets were used to record their activity. Average levels of moderate-to-vigorous activity fell from three hours a day at age 9 to less than an hour at age 15.

Nader said he was "surprised by how dramatic the decline was," and cited schools dropping recess and gym classes and kids' increasing use of video games and computers as possible reasons.

Two personal anecdotes: We didn't have computers and the Internets when I was a kid, back when the earth's crust was cooling, but I can assure you that any kid who wanted to avoid activity and exercise had plenty of options for doing so. My preferred method for staying inside and sitting on my butt was Strat-O-Matic sports games. I wasn't a fan of that newfangled thing called Dungeons and Dragons, but I knew people who were into it. I did actually play a lot of pickup sports back then, but believe me, if I wanted to be sedentary, I had ways of doing it.

And by the time I was 15, my main form of exercise was commuting to and from high school. That meant taking a bus, the ferry, and the subway every day; it also meant a lot of walking, including a fair amount of stair-climbing, thanks to the subway stations. You want a good workout, try hauling yourself and a backpack full of books from the South Ferry station (three stories underground) onto a ferry boat (one more flight of stairs in the terminal) at full speed so you don't miss the boat and have to wait around with nothing to do for another 30 minutes. I realize that's a unique experience, one that I was lucky to have, though I doubt I would have seen it that way at the time. But I do wonder: How many kids today are being driven to and from school now, compared to when this study started? Maybe that's an option for getting some of these kids more exercise that needs further exploration.