Supervision of outdoor play

I've been thinking...at what point and age can kids play in the backyard alone? Austin all of a sudden wants to be outside all the time! I don't mind going out a few times a day but Vivian and I get bored after a while. We literally have nothing in our backyard for him to play with expect toy mowers, baseball stuff and bubbles. I called home the other day and asked Nick what they were doing and he was inside cleaning the closet while Austin was outside playing. I kind of freaked but it doesn't take much for me to freak. Since then I have gone back and forth with the issue. What if there was a snake and he picks it up, or he opens the gate and goes in the front yard? There are lots of what ifs I guess. So far, I have decided that he can be outside in the backyard if I am with him, or if I am in the kitchen and can see him from the window...no sneaking inside the doghouse to pretend he is a puppy and no playing on the side of the house where I can't see him. I am curious as to what other parents have decided about outdoor play and supervision. This is all new for me. :)