In my opinion, it's not so much a matter of age but of safety. There is no age that is too young to free range, exactly, but if there are predators around that will hurt them when they're small, then you need to wait until they're older. I expect it also depends a lot on how your property is set up.
For me, i used to free range my chicks starting at 3 weeks or so (unless they were with their broody hen like SP said). But then my dog got some motherly instincts that call her to "bathe" peeping chicks, which often kills them. So now, i have to wait until they grow bigger, or isolate my dog while the chicks run.