See recent updates (e.g. 2/10/11), but also click on the Home Alone label in the sidebar or the bottom of this post.
Some of the most frequently asked questions by parents (of librarians!) are.
When can I leave my children home alone?
How old do my children need to be before I can leave them alone?
How old does my oldest child need to be before I can leave him/her alone with a younger sibling?