Probably, the predominance of cars in our modern societies (and cars always getting bigger) is one of the factor that yielded to observations made in this article.

But in some places (for instance where I live, Lausanne Switzerland) it's becoming easier to escape this car culture. I don't have kids, but some colleagues do. One of them does not own a car and her 2 children (6 and 9 years old) do everything by bike : going to school, to the activities (they climb!), etc. They surely look much more agile and risky than other colleague's children. It's quite crazy to see these two kids on their bikes, and how much liberating it is for them to move by theirselves. Unlike most kids who just commute in their parents's car.

Of course, I am aware this works only because there's a really good bike infrastructure. And people drive pretty well too, I guess.