Even in the rough and tumble NY of 40 years ago, parents allowed kids to roam freely up until the tragic disappearance of Etan Patz in '79. [1]

Seems like that event and the subsequent media fearmongering is what really spurred the cultural shift towards helicopter parenting and a reduction in youthful independence in the US.

[1] http://www.theatlantic.com/magazine/archive/2014/04/hey-pare...