We moved into a house where I let DD out in the back yard unsupervised (with an ear out for what was going on) just over 2 years ago. So she would play on her own out there from about 14 mths. She could get up and down the 2 steps without problem and the yard was fully enclosed with 6 ft high fences. There was a hazard at the back with a drop down into the neighbours yard (kind of a slope, though, for a metre) which she did get stuck down once (just before she was 2), but she yelled out for me and I rescued her! Otherwise, I would often let her go and she has proven herself pretty capable. In our new house the front yard is the playground area and I just make sure the gate is shut and padlocked. Here there are good fences and nothing that I think she could get into at all. I think she likes the opportunity to explore without me hovering. She's naturally a very capable child and aware of her own physical capacity.