You are the parent, so its up to you. How capable are your kids, have you been giving them increasing levels or freedom as they grow?
You just have to do what feels right for you and your kids, only you will know what that "age" is.
I have a 5 and 7 year old. They are out of my sight playing with friends down the street a lot.
They are within ear shot and sometimes I can walk to the edge of the yard and get a visual on them.
If they don't come when I call or...