Chickens can live well into their teens but rarely do. Maewest was 14 months old when I found her dying in the nest box the other morning, lying quietly, head down. She was the crazy hen, frantically afraid of being handled from the day I brought her home, though most hens can be easily trained to come take food from your hands, be picked up, and even sit calmly on your lap. She was always jittery, launching herself after the others with a piercing squawk, last to find the doorway of the pen when it was time to go back inside. Yes, after more than a year of going in and out the door.
I thought she'd expire last year when she prolapsed and I had to grab her (oh, the screams!), wash off her innards and stuff them back inside. The directions for this procedure found on the 'net do not include advice for restraining a struggling chicken and her big, clawed feet. Since the cause of prolapse or egg-binding in laying hens is often lack of calcium, I gave her extra calcium by dissolving tablets in water and coating the scratch feed with it. I think she wasn't metabolizing it properly, since all my other hens have been fine with the oyster shell calcium I provide in a cup throughout laying season.
Given her defects, I wasn't surprised at her shortened life. Not a lot is known about the way chickens feel pain. Years ago, people used to think they and most other animals didn't feel pain at all, simply because they didn't express their reactions the same way we humans do. Everything that's alive wants to hold on to life. I've seen the struggle in the tiniest scrap of a day-old bird. I've seen those little birds die and I've seen people die-- that quiet, sudden moment when they're gone and you struggle to comprehend what really happened. With or in this case without grief, it's a fearsome moment every time.