I think that nine year olds are alright for 1/2 hour or so home alone if their maturity is at nine years of age.
I agree, though for more than 1/2 hour would be pushing it I think. I have left my 9 year old home (he's very mature) for 15 mins and no more than 1/2 hour to check the mail/pay rent/ or whatever. The thing is I didn't want to do it at the beginning and it's very rare but he's feeling more independent and I want to help him learn more of his independents. I won't leave him there alone with my daughter (she's 3). And he gets to keep my cell phone during that time and we have very good neighbors that I also let them be aware when he's alone. If we didn't have neighbors that could step in I don't think I could do it.
Another thing is, sometimes I can't make it back home before my son gets home from school. He has strict "rules" for when I don't make it home in time. I've only not made it back in time 2 times from where I had to take my daughter to the doctor, and I was only late by about 15 mins. He LOVED it when I wasn't there because as he said "it gives him a chance to prove that he can do this".
One of the reasons I try to "let him loose" a bit is because my brother was so held down that he couldn't do much on his own when he was 16!!! He was 16 and wasn't aloud to use a knife to cut a bagel!!! He's almost 20 and now has mom/dad/sister do all his work for him. I think it's cause he's the baby and the only boy of 5 kids. When I was 9-10 years old (being the oldest) I was left with my other 3 sisters babysitting. Most times I was ok with it but other times I hated watching my bratty sisters. (You know how siblings can be)
