I don't think there are any hard and fast rules. It is not just the child but also the environment.
In a place where there's a lot of crime and kidnapping, any age is not safe, simply because even an adult is not safe. Secondly, every child is different. Some child mature faster than others. Also, in primitive tribal societies and among people who are very poor, the child is allowed to roam at a very tender age, maybe even 3-4 years old.