Chickens are only a half-step above grass on the food chain. As a result, they are pre-programmed to hide illness from everyone and anything in their world. By the time an illness or injury shows itself, it's often too late for us to "fix" the issue easily. Those are the birds we lose early. If they stay healthy (much of which is completely reliant on how well we care for them and their lodgings,) they can live much longer lives.
I have definitely found that hatchery mixes, intentionally bred for egg or meat production, tend to have much shorter natural lives than my purebreds. The "barnyard mixes" that pop up on their own in my flock tend to be the healthiest. There's a lot to said for natural selection. I have a Silkie/BJG mix that is still laying a huge brown egg every two-to-three days at almost five. We were also told that our Nankin bantams would only live four-to-five years, but my oldest hens are going on eight, as is my flockmaster and he's still ... ummm ... productive!
I have definitely found that hatchery mixes, intentionally bred for egg or meat production, tend to have much shorter natural lives than my purebreds. The "barnyard mixes" that pop up on their own in my flock tend to be the healthiest. There's a lot to said for natural selection. I have a Silkie/BJG mix that is still laying a huge brown egg every two-to-three days at almost five. We were also told that our Nankin bantams would only live four-to-five years, but my oldest hens are going on eight, as is my flockmaster and he's still ... ummm ... productive!