Age children walk alone?

Around what age do children generally go off on their own (i.e., without an adult)? I saw a young child once all by himself about to cross a busy intersection and was horrified to see him alone, yet I know Japanese kids often do go off by themselves. And are even capable of getting to their destination. But still! He must have been really small for his age, because he looked like he was younger than he probably was. Or maybe not? (I can't imagine he was older than 4 years old.)

In the past, it was quite common for children age 5 or 6 to walk alone to and from school, even take trains or buses.

You still see this today but the rise in crime in Japan (or better explained, the rise in media coverage of violent crimes) has led people to be more cautious about letting their kids walk alone.

Now school stress that kids move in groups and also PTA "patrol" activity has increased.

i can say children over here starts going out by themselves when they start elementary school...i often see 6 or 7 years old kids taking the bus or walking to school without parents...(i live in Chiba,in tokyo i see less of these cases)..

btw i think that the parents should take more care of their children,now that the criminality is increasing in japan...moreover,the traffic street are dangerous..just few days ago a 7years old kid died not far away from my house,hit by a bus!he was coing back from school,of course alone.

And also,i notice mothers who leave their 3 or 4 years kids running in the parking lots,or in crowded streets,or in the shopping malls,without looking at them...

I'd like to think it starts from the age when kids can physically walk around alone without a stroller and navigate by themselves. It's not uncommon to see preschool/kindergärtners walking by themselves or taking trains. Despite the rise in crime, especially crime involving very young victims, it hasn't changed much.
Also, there are many families with single working mothers or where both parents work that forces kids to get to/from school alone and there is a lack of babysitting culture, daycare centers, etc. It's a pretty big problem and from an American point of view, exposes children to needless risk not only from crime, but cars, bicycles, irate dogs, construction site too.