Before you let them out, just bear in mind that they have feathers where their brains should be. Sometimes you can pen them for a short period and they stay forever, sometimes you can pen them for a long time or hand raise them and they still leave when released... Glen's right, in that 6 weeks minimum is a good starting point. If your birds are in the barn only, with no access to view the territory they will own when released, they may not recognize where home is, or be so excited to be outside that they take off and forget where they started from.
They can eat non-medicated food, but the chicken food usually lacks the protein they need. You may try offering them cat food (that is what we do), just the dry kibble... ours love it. They are jerks and pick through their feed bin eating all the kibble first.