To your point, I think 4-6 years is probably the norm for backyard, non-production hens. Prior to the relatively recent craze in backyard chicken keeping, the hens would most likely have been processed as soon as egg production or quality dropped, unless it was a pet or favorite. All things considered equal across the flock members, I think 7-10+ years primarily comes down to the individual hen, with care, climate and other factors being secondary.We lost a buff orpington last fall at 4.5yrs to cancer. We tried treating for EYP for a few days just in case, but the vet was fairly confident in his feeling a mass. He's one of the few vets locally that specializes in birds, so I trust his expertise. He told me that in his experience, most all of the hens he sees over age 4 have cancer, usually reproductive. It doesn't always kill them, but it's started somewhere in their body.
Humans have bred chickens for many, many years, but if we think about it it's probably only been in the last hundred years or less that we've desired long lived birds. Historically, humans wanted egg layers, then used the bird when egg production drops. I'm sure many hatcheries still breed for maximum eggs, not for healthy, long lived birds.
Interestingly, domestic rabbits have the same issue. Historically bred for production, not health. Also super active reproductive systems that get cancer fast. Spaying or neutering a domestic rabbit takes their average lifespan from 4-5 years to 8-10 years.
I'm currently battling suspected EYP in our 5 year old barred rock. Like many other posters in this thread, reading about 8-9 year old hens makes me feel like I'm doing something wrong.
Question for readers: what's your climate like? Do folks with longer lived birds live in less extreme climates?
Our climate is high desert. Dry, but seasonally can go from 110F to -10F. Our oldest bird is 5.
We live in Colorado and our girls see some crazy weather swings. They seem to thrive on it, lol.