Old Enough to Stay Home Alone

When I was growing up, it was not unusual for children to be left to their own devices. I was given a house key at 8, when my mother returned to work. But that was a different time, and in my case, a rural town in the mountains.

What is the appropriate age for a child to be left, unattended, in Milford? A few weeks ago, left alone in a car, a child who was waiting for her mother, as it turned out, and who was 10. Isn't it okay to leave a 10-year-old by themselves? 

When is the right age to leave kids without a babysitter? Or is it a case of the maturity of the particular child, and not the age? Please share your thoughts.