When I was 10 my parents and grandparents thought I was old enough to stay home alone. But then after almost cutting off my thumb on a power saw (I believe I stopped the bleeding by promising God I'd stop being mean to my sister) it was awhile before I was home alone again.
In general though, it is a question of maturity and therefore boys and men should probably never be left alone but girls are probably fine around 8.
See NPR: Home Alone: Is Your Tween Ready?
