How Old Is Old Enough To Play Outside Alone?

I live on top of a hill.  There's more than 500m between my house and the street, and it's really a kid's paradise up here:  a playset, a trampoline, our neighbors' regulation soccer field.  

Still, as safe and secure as we are, we wonder at what age it's safe to let the kids play outside without adult supervision.  I've let my oldest daughter go outside with her friends since she was five, but what about younger siblings and cousins who want to tag along?  What if the kids have a fight and storm off, and get lost?  

I can't even imagine how parents make this decision when they live in neighborhoods, or cities – places where cars and strangers are a real risk.

Rosa Brooks, in the Los Angeles Times, laments the fact that parents won't let their kids play unsupervised anymore, like they did when we were kids.  She points out that children have a greater chance of being struck by lightning or of being killed in a car accident than of being snatched by a stranger or getting hurt while playing outside without adult caregivers — plus, such free, unstructured play helps teach kids independence, resiliency, maturity and a host of other skills that may be slower to develop if a parent is constantly hovering.

I get it.  But I also understand the parents who think, "Yes, the risk of something horrible happening is low.  But if something did happen, I'd never be able to live with myself."

So, what's your philosophy?  Are you like Lenore Skenazy, the New York City mum who sparked an enormous controversy last March when she armed her 9-year-old son, Izzy, with a MetroCard, a map, $20, and c for payhange phones and sent him off to find his way home alone on the subway?  Or are you more cautious?