Well, I have seen kids I know... who are like 9 and 6, riding their Razor scooter from their home... and to the store like a mile away.
NO parent in sight.
Then I will be at this same store in the neighborhood, and the kids are there, by themselves, and in the store or cruising around the little strip mall there. By themselves. No parent in sight. I know who their parents are... and have never seen their parents, anywhere near them nor in the vicinity..
And then I see them just cruising around, looking bored, and just cruising around with their scooter. By themselves. This happens, regularly, since school has ended. I have seen them by themselves, several times.
I talked to the kids once "where is your parents?" They reply "at home..."
Hmmmm I say to them, and be careful, you're quite away from home.
They are not the type of parents, that like having stuff said to them or about their kids. They are, bull in a china shop types. Nor is their home anywhere near, the store and strip mall their kids are at frequently. Nor in view of it.
For you, being you are outside and you notice these kids also outside and no parent routinely in sight, well then you keep an eye out... and that makes you comfortable. So that's fine. You are not responsible for them per say. But, like that TV show "What Would You Do..." if you notice some harm to them or whatever, then call the Cops.
Every family is different. And per supervision or not of their kids when outside, no matter what age.
It has to do with age and independence and being hovering or not... and just common sense. A combination of things. And there is no right or wrong. It takes about 3 seconds for a kid to be abducted. And some say these things are not common. But the bottom line is: do what YOU feel is safe and needed for your children. Regardless of age. It is your kids. And if you are the type that notices unattended kids out and about, regardless of age, then you make a mental note. But that does not mean you have to go to their home, kids in tow, to tell the parents or to tell them what to do. Things like that, are not taken well... by the other parents.
My kids play outside. I am inside. Or outside. Or both. But my yard, is enclosed by a wall. And if they are outside in the front driveway... I keep my front door open, and they have walkie talkies, and they listen to what I say and are loud kids. And also though, we know ALL our neighbors on all sides of our home. And they know us. But I still go outside and "check" on my kids here and there.
I don't let them however, ride their bikes/scooters/skateboards around the block without me. Even if they are 5 and 9 years old. There have been car accidents, in my neighborhood.
Nothing can be predicted, no matter how mature or smart or obedient a kid is. Right? And per their ages, you do what you feel is adequate or not, to look out for them.
There is no absolute "rule" that says only young kids get harmed, and that older kids are responsible & obedient enough and smart enough and therefore can be alone. Even college kids/girls.... here in my city, they get accosted or worse, while just walking to class. Random things.
So many scenarios.
The best you can do, is what you think is best for your child or children, using the common sense that you have as a Mom.
And that is fine.