In our yard, under 5. My child has never been the type to risk her life in play. She's just not that kid. Her outside play involves a ball, hula-hoops, the trampoline and swings... nothing especially dangerous and nothing that requires regular supervision at this stage. There are no pets either and we've got a 6ft fence surrounding it.
Outside of our yard... well, prob over 10. She's has zero common sense and zero sense of "street smarts," (I know she's only little, but other kids seem more street smart at that age than she is). She would probably freak out if left on her own to play away from our yard anyway.
Plus, she's an only child. It's not the same as having a handful of children I know all playing together... it's just her. She could so easily be snatched up and taken away without me, or anyone else, knowing. Perhaps if she had siblings similar in age, or neighbourhood children who were trustworthy and "street smart," it might be different.
I just don't see her being in a place where she's capable of being unsupervised in public place until after 10.