I wouldn't let my little one roam free - she will be 10 in a few months. She could find a way to get into trouble. Not only that, but as others have mentioned, if the kid looks younger than her age, someone may call the authorities on you. Kid finally doubled her BMI this year, so she is now 2nd percentile. She is both lightweight and short, looks like a 7 year old.
I would not leave my 16 year old home alone overnight. Nor would I leave my 18 year old home with the 16 and 9 year olds. Even if your kid doesn't do anything wrong, others may find out that the parents are gone. Many years ago a neighbor left their kid home alone - she was probably 17. She was a good kid, but she had a few friends over. These kids were fine, but others heard that the parents were gone, and tons of kids she did not know showed up. There was a racial incident on the front lawn.
When I was in HS, my boyfriend's parents left him home alone for the weekend, and he wrecked a very pricey sports car. Too difficult to explain here how he had access to a car of someone he didn't even know, and he was fortunate that he wasn't hurt and the car owner was cool about the whole thing. These kids went to good colleges and at least one went on to get a graduate degree from a respected university, so they weren't dumb kids.
Too many things could go wrong. I will let her play on the playground out of my sight at her sisters' sports tournaments, but only if multiple kids are involved. If something happens to one kid, another can go for help. A couple of years ago, she was playing with these kids (younger siblings of my older kids' teammates), and a kid sprained his ankle. My kid was able to go get help. And these tournament sites only have player families there - typically there are not random members of the public wandering onto the playground.
And finally, while my state does not seem to have a minimum age to be left home alone, I have seen plenty of cases on the news where parents were arrested for leaving them. Some were cases where the eldest seemed old enough to be home alone, but were left in charge of younger ones.
