left at home daytime is way the fuck different than overnight, IMHO.
But yeah, 9 YO for me, because my mother had no choice in a rural environment where she had to work until about 6PM each day in a town 40 miles from our house. My kids never remotely had the same situation.
The 'feral kids' description up thread is perfect. I'd ride my bike to my buddies' houses and hang out, or grab the pellet gun/ .22 and my dog and go plinking on about a 500 acre farm, with no limits to stay on just our land. Lots and lots of pickup basketball/ baseball/ football games were had, or we'd swim or fish. You stayed busy, or at least kept up the appearance of being occupied or the adults would starting finding shit for you to do( AKA work.) And whatever house you were at, you were considered one of their kids when chores and shit were handed out. Nobody sat and watched their buddy get hosed. LOL.