More fearmongering on the backs of children

[firecat]

"Study: When kids become teens, they get sluggish," by LINDSEY TANNER, AP Medical Writer (warning, contains fearmongering)

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

I've seen a few critical comments on this study along the lines of "Teens have more responsibilities," "Schools make children hate exercise," and so on. But what I'm wondering is — if somewhere between 10 and 15 is the magic cut-off point where children become less active, has anyone asked whether there might be a biological cause, something to do with puberty? Perhaps the body says to itself (via hormones) “We have to save some of this energy for reproduction now.”