Yet another aha!
According to recently published research by the American Medical Association, studies show that kids' activity level drops after age nine. Seriously drops from, on average three hours a day to about a half hour (on weekends).
According to the NYTimes, "The findings, which measured everything from moderate walking to vigorous athletic pursuits, show clearly that even the most energetic young children experience a precipitous drop in physical activity as they reach puberty."
I guess they just can't help it.