Ds is 9, turning 10 this week, and we just started leaving him alone in the last 3-4 months. He doesn't like to be alone in the house, so it's not very frequent. He's becoming more comfortable with it. The first time we left him, we were gone for about 45 minutes (he was getting over being sick, his sister had something she wanted to do, and dad had a meeting). Most times it's about 15-20 minutes tops.
Before I let him stay home alone, I confirmed that he knew how to reach us on the cell phone. I made sure he knew the rules about answering the door (don't do it) and answering the phone (let the answering machine get it). He was under strict instructions not to use the stove or anything else possibly dangerous and to call us if he needed us. He spent the entire time playing Wii and didn't even notice!
I'd be comfortable leaving him home alone for about an hour. After that I think he would be uncomfortable. He's a very cautious and responsible kid. (I remember that my mom went back to school when I was 10, and I came home to an empty house and started dinner. So, 10 can be mature enough.)