Some legal opinions on the age when it becomes ‘okay’ to leave kids unattended:
http://www.latchkey-kids.com/latchkey-kids-age-limits.htm
http://family.findlaw.com/parental-rights-and-liability/when-can-you-leave-a-child-home-alone-.html
http://www.kidsandcars.org/state-laws.html
http://www.nspcc.org.uk/preventing-abuse/keeping-children-safe/leaving-child-home-alone/