At what age do we stop being so active?

I wondered, at what point in our lives have we shifted from running to walking? From moving a lot to moving only if we need to? Basically, when does our moderate-to-vigorous activity level shift from the tireless running allover the place to the more adult-like style of moving when we need to? A very interesting study, published in the July 16, 2008 issue of JAMA gave some insight. The authors analyzed the data of more than 1000 children, almost half of them were boys and the other half were girls. The researchers followed their patterns of moderate-to-vigorous physical activity from age 9 to age 15. They found that at 9, the average child engaged in good 3 hours of moderate-to-vigorous physical activity, which is well more than the recommended minimum of 60 minutes per day. By age 15 years, adolescents were active at that level for only 49 minutes per weekday and 35 minutes per weekend day. Boys were more active than girls. It is well-known that decreased physical activity is an important factor in childhood obesity.

I am not sure what exactly happens. Why at some point in our lives we start walking if we don't have to run, and sit if we don't have to walk? At any rate, knowing that the transition takes place between the ages of 9 and 15, we can target that time interval and aim at keeping children engaged in organized moderate-to-vigorous activities during that period, hoping that the habit continues with them for so many more years of their lives.

Reference:

Nader PR, Bradley RH, Houts RM, McRitchie SL, O’Brien M.
Moderate-to-Vigorous Physical Activity From Ages 9 to 15 Years.
JAMA. 2008;300(3):295-305. (Abstract)

Affiliations of the authors of the reference article: Department of Pediatrics, University of California San Diego, La Jolla; Center for Applied Studies in Education, University of Arkansas, Little Rock; Statistics and Epidemiology, RTI International, Research Triangle Park, North Carolina; and Department of Human Development and Family Studies, University of North Carolina, Greensboro.