This is my ethos. I have two young boys--one 4 year-old and one 18 months--and this is how they are growing up. My four year old was riding without training wheels by two and on skate ramps by three [1]. My wife was initially against it but through my stubborn insistence, she's come around. As I mentioned in another comment, my sister and I grew up doing all sorts of dangerous shit and I will forever be grateful to my parents for not raising pansies. We build a rickety tree fort high in a tree and threw balls of mud and rocks at each other and the other neighborhood kids who played with us. We rode our bikes across major streets, we made homemade fireworks, and we had a trampoline in the neighbors' yard and a skate ramp in the driveway. It was an amazing childhood and it's exactly what I want for my boys. When we moved out here to small-town Kansas from the Seattle area, we spent time looking for the right street and we found it. Packs of kids roam unsupervised and there are few backyard fences. It's perfect. I hope that "playborhood" does become a thing again because there's no better way, IMHO, to raise kids.

[1] https://www.instagram.com/p/BHNBJqmhM5T/?taken-by=christophe...