I had a bantam cochin hen that lived to be about 16, give or take a year. She didn't lay eggs after a certain point, but we never got rid of her. She served me well through the entire time I was in
4-H. All the other birds we owned suffered the fate of the first responder to your post... they didn't live long because they got eaten by something first. I imagine life span would vary by breed. Just as it does in dogs and cats. But with clean secure living quarters and minimal to nothing for medical care, they seem to live quite a long time.