There is';t any clear rule on this, as this decision is typically lef to the parents. Most jurisdictions suggest that a child should never be left alone of under the age of 12. When my own children were recently closing in on that age a dialogue was started by them as they asserted their independence. I then took an informal survey of judges in Los Angeles County I know and most indicated that they would have a big problem if a child under the age of 14 was left home alone, so that's what I went with. I think it's a good rule of thumb.
Many factors such as the child's age, maturity, intelligence, etc. may figure into this determination. I'd tell my clients in my state that 14 is the age that judges seem to not get cranked about, but that is just an approach I have used. There is no mandatory age in my state.
Good luck.
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