I distinctly remember babysitting when I was 10. For a two-month-old. Who was not related to me. Looking back, I can't believe those parents left me alone with their baby! As a parent now, I wouldn't even consider something like that. I think each child is different in when they are ready to stay by themselves (based on their maturity, intelligence, self-policing, etc). And of course, the circumstances matter, too. Such as, how long the parents will be gone, are there neighbors nearby, is it daytime or nighttime. This is something each parent has to decide for each of her children. It's not a one-size-fits-all solution. In fact, the rules may even vary for children in the same family.