I'm solidly in the Millennial bracket and the closest thing I had to "planned activities" before I was old enough to pick them myself was Little League during the summers.

Then again, I grew up in rural Iowa, and I've heard cultural trends take a decade or so to make their way from the coast all the way inland.

But I think some of it is also just a matter of what you are focusing on. I'd guess there are plenty of kids growing up similarly unsupervised as in the past, they are just more out of sight than before.