The bulk of the chickens in my flock are in their fourth year - well past the age that most commercial laying hens are allowed to live. And I know that chickens don't live forever. Over the millennia that chickens have been domesticated, high egg production was the trait that was valued above all else—so that's the trait that was selected for. Longevity wasn’t even considered, since chickens typically were slaughtered long before the end of their natural life. Thus, chickens don’t have long lives. Laying an egg practically every day eventually wears a hen out. It is highly probable that eventually something will go wrong with her complex, high production egg-laying machinery—the oviduct becomes infected; an egg becomes impacted; the oviduct breaks and leaks yolk into the abdominal cavity which becomes infected; tumors form—the list goes on.
When I was a kid on the farm, the problems of aging hens was not a problem because there were no aging hens. Chickens raised for meat were slaughtered in their first year. Laying hens were kept for two years and when their egg production slowed they became stew. Old chickens and their health problems became a reality only recently, when people like me started keeping small backyard flocks. We backyard chicken people bond with our chickens and our rationale for keeping them goes beyond eggs and meat. We keep them for the pleasure of keeping them, and for the satisfaction that comes with nurturing them and giving them a good and happy life. Chickens have become pets. And that’s OK—chickens are fascinating, compelling, and beautiful animals. But the grim reality that is interwoven with the many pleasures of keeping a flock of backyard hens is the anxiety and angst of dealing with the inevitable sick birds and the anguish when one of those sick birds dies.