Nine year-old children are active for more than three hours a day - but by age fifteen, individual activity levels drop to less than 45 minutes per day, according to an article in the Journal of the American Medical Association.

Researchers found that only a third of teenagers got the recommended minimum one hour a day of aerobic exercise, and that age 13 was the year that activity levels dropped off dramatically. Things got worse on weekends, too, when activity decreased from 49 minutes a day to 30.

The scientists speculated that older teens tend to watch television or play videos with friends, rather than pursuing active games.

"I was surprised by the degree of the drop. It’s a dramatic shift," said Dr. James Griffin of the National Institute’s Center for Research for Mothers and Children. "Younger children appear to be naturally active, but as kids get older, they find fewer opportunities to be active."

The study, led by Dr. Philip Nader, a professor of pediatrics at University of California, San Diego, tracked over 1,000 American children from 2000 to 2006, providing them with devices that recorded their movement.

Labels: activity, exercise, teenagers