Depends on the child and the yard.

Because my recently turned 3yo has an older brother (5yo) who I can trust to keep an eye on him and let me know if anything goes wrong I let them play out the back all the time... loosely supervised. However I also find it easy enough to let this happen because we have floor to ceiling windows all along the back part of the house making visibilty really good. Even sitting here at my 'pute I can see them. If I didn't have these windows I would be a bit more nervous about unsupervised play.