This LA Times story (by Rosa Brooks) about the erosion of free, unstructured outdoor play and movement for kids really hits the nail on the head: little kids are just not allowed to "go outside and play," and we treat big kids who do as potential threats to be moved along as quickly as possible:
But today, for most middle-class American children, "going out to play" has gone the way of the dodo, the typewriter and the eight-track tape. From 1981 to 1997, for instance, University of Michigan time-use studies show that 3- to 5-year-olds lost an average of 501 minutes of unstructured playtime each week; 6- to 8-year-olds lost an average of 228 minutes. (On the other hand, kids now do more organized activities and have more homework, the lucky devils!) And forget about walking to school alone. Today's kids don't walk much at all (adding to the childhood obesity problem).
Increasingly, American children are in a lose-lose situation. They're forced, prematurely, to do all the un-fun kinds of things adults do (Be over-scheduled! Have no downtime! Study! Work!). But they don't get any of the privileges of adult life: autonomy, the ability to make their own choices, use their own judgment, maybe even get interestingly lost now and then.