No matter their age, you will need to protect the chicks from ground predators, for example with fencing (electronet mesh fencing is a good option and has step-in posts and is easily moved), or some people rely successfully on dogs. Protection from aerial predators is more difficult. You can provide shelter, or string fishing line in the yard to slow hawks. There is a section in ATTRA's Alternative Poultry Production and Outdoor Access http://www.attra.ncat.org/attra-pub/PDF/poultryoverview.pdf on protecting birds from predators.
Here is one producer's experience: "We raise free range birds here on our pasture. We do lose some to hawks, although our Pyrenees keep the coyotes and other four-legged critters away and try to chase off the hawks. I have a chicken tractor that is covered with chicken wire from above that we can move around the pasture. Size-wise, there are pretty big at 8 weeks but still no match for most predators I suppose. I would try just letting them out at mid-day and see how they survive an hour or two. Ours could always get back in the chicken tractor, which is inside another fence. The Pyrenees do an excellent job of protecting the birds.
