In your opinion how old should a child be, to be left alone at home?

Maturity and responsibility is the key. However, each state and most areas of the country have "legal age" the child must be before the child can be left home alone or in a csr alone. Regardless of the maturity level of your child check the age in your state. Here the child has to be at least 8 (how they come up with the age is beyond me).

Depends on the child certainly; my older sisters were allowed to be home alone before I was. If I recall, I was home alone for the first time when I was 13 (my sisters, 10-12). That said, I grew up in a safe neighborhood and knew many of the neighbors, and having three older sisters meant that I rarely had a chance to be home alone. The first time I was home alone for an entire night I was already 16, and that was a rare opportunity. I loved it.

It depends on the child. I know some 13 year olds I wouldn't leave home alone, yet I started babysitting at that age. It also depends on how long you are leaving them alone. I think most kids can be left for an hour or so around the age of 11 or 12.  A few hours around 14. 

At least 13 or 14, but only for a few hours. Not an entire day.

I'm 14 and I've been left alone at home before a couple times. Usually not for very long, but one time it was most of the day. I think I've proven to my parents that I can handle it. We do live in a gated community and everything, so I don't think they're worried about that kind of thing, but they're also not worried I'm going to throw a party or burn down the house when they're gone. :P

I'm not trying to brag, but I've always been pretty indepdent. I know how to cook, do laundry, and even iron, and how to set the alarm too, and I know most of my friends don't do those things. So it depends on the kid :D

It depends on the child and the situation surrounding the decision. Plus I think cell phones are an added bonus now a days with every kid having one.

As Walter pointed out, it depends on the maturity of the child. When my son was 10, I left him for an hour while I went grocery shopping. We have great neighbors so I knew if he really needed help, he wouldn't hesitate to go next door. He was in his teens before I would leave him for more than just a quick trip to the store. However, since he regularly did sports, he was out more than he was at home.  :)

15 But it really depends on the advancement

of the child.

Actually it depends on how mature and understanding they are, some 10 year olds are so mature that they can be left alone all day and won't cause trouble or get into any whereas some 15 year olds are so immature that they just can't be left home alone, but still I would say as a general age to be 14 and above .