As a legal definition, this varies from state to state, though some states have no statute in this respect. The answer can be determined only by the parents of a child, based on the individual maturity level of that child.
Every state has different statutes that apply to how old a child needs to be in order to be left alone in the home. For example, Illinois requires that children be at least 14 before they are left alone at home. Meanwhile, states like North Carolina and Maryland require the children to be only 8. Further, some states have no minimum age at all. In the end, only a parent can decide if a child is mentally and emotionally ready to be left on his or her own. The best means of determining this is simply trying it out. A 20-minute trip to the store for eggs and milk is a good test run, and it may take several of these for a parent to feel comfortable with the idea. Parents should be sure that their child is aware of what to do in the event of an emergency and have supplies on hand to address any possible situations that might come up.