I said 10, I'd trust Keira (8) now and Cooper(5) for that matter, with Keira. They are usually up a couple hours before me and Harper, like they get up at 7, and I get up around 9. Sean feeds them before he leaves for work and they play in the basement or their rooms. I wouldn't leave them for long periods. But Cooper would be scared.
When I was 9, stayed all day by myself, and I loved it! LOL!
I've left them in the car, only if I can see the car from where I'm at. Like if I had to run into walgreens or the post office. But my van, you can take the keys out, lock it and turn it back on. I've only done that because Harper is so dang heavy for just 5 minutes in and out of somewhere.
I've never left them at home alone though. And probably won't anytime soon.