Its funny, because I see young-ish kids alone or in small groups walking around all the time in Brooklyn. Between 2:30 and 4:00 PM the sidewalks around my apartment are choked with unaccompanied kids.
I've offended a lot of suburban friends and family though, because they don't understand how we could raise a child in NYC let alone give them any kind of independence. I've been told that it's irresponsible, that I'll be endangering him, and that I'm crazy to think the city is a safe place for a child, among other things.
With all due respect, whoever is telling you that is should mind their own fucking business.
We live in Hong Kong and think that it's a great environment (albeit insanely expensive) to raise our one-year old child but heard the same reaction to friends and family. Some people get obsessed with the idea of safety and can't understand the value of freedom to explore, easy access to culture in the form of theatre, art, museums, ... Instead they only optimise for that single dimension of safety and are offended whenever someone challenges their obsession by making a different choice than them.
Time for some new friends...
These problems are not unique to suburbia, but they're definitely amplified.
There's packs of them that transit their way to local skate parks near me.