This is a question that I am frequently asked, and a flippant answer might well be: as long as the flock keeper decides they should live.
Many people think that chickens are short-lived, and in our modern, factory-farming approach to eggs and chickens, they are: the average layer is culled at three years when her egg-laying naturally begins to taper off, and many layers may be culled as soon as their first molt occurs.
For people who prefer a less-utilitarian approach to their layers – backyard chickens can live for a decade or more. The average is about 8 to 10 years for a chicken that is healthy and kept safe from predators, although there are reports of pet chickens living well into their teens. Possibly one of the most famous long-lived birds of our modern era of backyard flocks was Victoria, a crotchety black rock hen, which according to an article in the Daily Mail in 2013, laid two eggs at 17.
Consideration of layer longevity cannot be divorced from the other question of how many eggs can a chicken lay? After all, the primary reason for keeping layers is to have a steady supply of fresh eggs. Like humans, hens are born with a specific number of eggs. How many they actually will lay over their lifetime is tied to their health, lighting and diet. Generally, during her first two years, a hen will lay about 200 eggs a year; starting in year three, egg production declines by 1/3 to 1/2 of her first year egg production.
The amount of light (natural daylight is always better, but not always realistic in Alaska) that a hen is exposed to affects her laying schedule. Again, like humans, the low-light days of winter signal to hens that it is time to slow down. Many flock-owners, commercial and backyard, maintain a steady supply of eggs through fall and winter by exposing their layers to long hours of artificial light (>12 hours). While this will keep most hens laying, it does not give their bodies the natural downtime needed to recover from the heavier schedule of egg-laying during spring and summer months. This may lead to earlier mortality and somewhat increased chances of disease susceptibility, however, this does not militate against the practice for flock-owners seeking to maximize egg production through the steady replacement of two to three-year old birds with new young ones.
In over 14 years of having a backyard flock, I have always kept my chickens through their natural life cycle, and currently have four elderly, but spry, birds – all of whom are 10 or older. As an additional, admittedly unscientifically-based plug for the benefits of keeping older birds, I have never had any of the terrible bad habits (cannibalism, feather-picking) that can plague backyard flocks. I believe this is because an all-ages, diversified flock (including a rooster) is more in keeping with the natural flock habits of chickens – which, like sled dogs, prefer hierarchal groups. It is my oldest hen, who at 11 is really the flock head – she keeps all of the other ladies and rooster in line. Although it is not a philosophy for everyone, it can be beneficial in effect to keep the older hens around, even after their peak egg production has stopped.