I think it all depends on the child. My oldest child was much more mature at a younger age than the little guy is. I knew I could walk outside and grab mail etc and he'd be fine at 3-4 years old. I didn't leave him alone alone until he was 12 and then it was just to run to the corner store for a few minutes or post office things of that nature where I'd be back in less than 30 minutes. My youngest one is into everything the minute you turn around (he's 3) and so I carry him with me to check the mail because I just don't trust that he'll not do something silly if I step out of the house for a moment.
when I was 10 I was left alone for an hour or so at a time and I did alright, but I think that may have been a bit too young for all that freedom lol I did get into things I shouldn't and stuff at times.