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| April 2009 |  |
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he conventional wisdom is that American kids become more sedentary
in their teen years. Now a major new study suggests that it’s
happening a lot sooner. Children’s activity level peaks at age nine, when they move around for about three hours a day, on average, according to a study published in the Journal of the American Medical Association. From then on, the decline is  steady
and, by age 15, kids’ average daily physical activity drops
to about 49 minutes on weekdays, 30 on weekends.“I was surprised by the degree of the drop. It’s a dramatic shift,” said Dr. Philip R. Nader, emeritus professor of pediatrics at the University of California, San Diego and an expert on childhood obesity. He was lead author of “Early Childhood Care and Youth Development,” a study for the National Institutes of Health (NIH). Dr. Nader and his team tracked 1,000 children born in 1991 at 10 locations across the country. They measured the kids’ height and weight at several points in their early years. Then they monitored each child’s activity for one week—at ages 9, 11, 12 and 15, using an electronic device to measure everything from moderate walking to vigorous athletic pursuits. Overall, boys tended to move around a bit more than girls. At ages 9 and 11, almost all of the children in the study were moving at the government-recommended level of an hour a day. By age 15, however, only 31 percent of the kids met that guideline during the week and just 17 percent on the weekend. As kids get older, they find fewer opportunities to be active, and 13-year-olds are particularly prone to slowing down. Though the kids in the study were not measured at 13, mathematical modeling showed it was the point at which daily weekend activity for boys and girls alike dropped below 60 minutes. Older kids spend more time playing video games or watching TV with their friends, says James A. Griffin of the NIH’s Child Development and Behavior Branch of the Center for Research for Mothers and Children. The researchers also pointed out that schools tend to curtail physical activity as children get older. Recess stops and many schools have dropped physical education as well. Sports also tend to become more exclusive, with only the better athletes continuing to compete. The period of this study, which began in 1991, coincides with the national “epidemic” of childhood obesity. The message to parents is to be aware of this research—and to find ways to help their children get involved in physical activities as they get older. 
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