When I was a kid the legal age to babysit was 12, but I'm not sure if it's still the same now. That was a long time ago. ;)
I do know that my neighbors daughter turned 12 a few months ago and posted signs all over the neighborhood, and went door to door, telling people she's now available to babysit. A few of the neighbors have already hired her. But, her mother is only a few houses away if anything happens.
I think it's okay as long as you know the people, and as long as there is an adult nearby that you can call on if something happens.
Check out this link, it may help a bit.
http://www.google.ca/url?sa=t&rct=j&...,d.cGE&cad=rja