I agree with the previous two posters. The typical backyard chicken kept in an enclosure with shelter and an enclosed run will live longer then a free range chicken. Chickens, especially hens, have very little defense mechanisms to protect them from predators. Hawks, owls, eagles, raccoons, large snakes, possums, weasels, fishers, dogs, wild cats, etc. all pose a threat to a poor defenseless hen.
I think it is common knowledge a cornish/rock cross typically only lives a few months before succumbing to leg or heart problems unless feed is severely restriced and plenty of exercise is available.. A battery hen kept in a small cage also has a decreased life span due to the conditions she is kept in.