The laws might be different in different states. Check with your local police station to find out what laws pertain to the state you live in. However, despite the law, parents and caregivers always must look at the child's maturity, the safety of the community, and access to emergency assistance. A child may be mature enough to stay alone, but not to care for younger siblings. If you are concerned that a child is left alone and is unsafe, call your county department of social services.
The age varies from state-to-state, the link below will provide you with the restrictions for each state. Regardless of age and what is legal, a child should also be mature and responsible and safe!
As far as legal actions that could be taken against the parents/guardians-I'm not sure, neglect and child endangerment come to mind but may involve more if an accident occurred at the home or a crime was committed by the unsupervised minor.
I am a very protective parent and I am very particular about the safety of my children.
When my eldest son turned 14 and my daughter turned 13, I started considering them to stay at home alone. I was worried they will encounter an emergency and they don't have anyone to call on immediately for help. While reading an article on a blog, it mentioned that there was a service I could use to track my kids to be sure they were always in safe places. At the bottom it said I could follow the site anationofmoms and be entered for a drawing of 6 months free of the service. Not bad! http://anationofmoms.com/2011/08/protect-your-family-giveaway.html.