DD was three when I first let her out by herself. I call it semi-supervised, because I still stick my head out every now and then and I can see a large portion of the backyard from my couch. It is a very child friendly backyard. DS is 1.5yrs and is allowed outside with DD while I stay inside - but like I said I can see almost all of the yard and if DS goes around the corner and plays in the dog water (which he loves to do) or anything else naughty, DD loves to dobb him in to me

I think it depends on what your yard is like as much as anything.