Sadly, times have changed from when children could safely play outside unsupervised. That said, the degree of supervision should be commensurate with the age of the child, the environment (dangerous features like the pond Made mentioned, lack of fencing), and any special conditions (like unsavory neighbors or an estranged spouse).
This thinking applies to walking home from school, or riding a bike to the store, or walking the dog, or any other potentially unsupervised activity.
I wish it wasn't so.
By the way, my kids are 31 and 26 and they can pretty much play outside by themselves.