I'm seriously not interested in keeping my chickens forever, and honestly the 20 some years quoted in the original posts would be incredibly unusual. 10 years would be a rare bird.
While I enjoy my chickens, and take very good care of them, talk with them, laugh at their antics, and name them, I also need their eggs for the family and must live on a budget. I can't afford to pay for birds that are not producing well as grain is expensive and my field rotation and space limited. There are a couple that will be kept until they expire of old age, as they have become endeared pets, and my brooding bantams will be treasured until their demise (and mourned at their loss), but most I sell or give away at 2 1/2 to 3 years of age, the time when most substantially slow laying.
In my experience, my commercial RSL play out by year 3 and are spent, even with high quality feed and care allowing for natural molts and rest cycles. Few have made it to year 4 or 5 as strong layers. Barred Rocks are better. My barnyard mixes are the best for overall longevity and productivity. But having said so, most of my birds have succumb to something by age 6 (usually predators or the slow wasting away of cancer).
And honestly, I am not interested in keeping older hens as they are much more susceptible to illnesses as their immune systems slow, which they can pass on to the flock. I value the flock and it is my duty to protect them. In nature, the older, slower birds are picked off by the predators keeping the flock younger and healthier.
I have learned there are many people who will gladly pay a reasonable price for a 2 year old bird, or take a 3 or 4 year old bird for free. I have a friend who likes the larger eggs of the older bird and has a large farm, so a majority of my birds are given to her when they turn 3 or 4 for a lifetime of leisure on her farm. We call it the Happy Hen Retirement Farm.
It has been a good solution for me so that I can keep egg production up, maintain a healthier younger flock, while yet letting someone who enjoys a flock of established birds do so.
But yes, I've also retired a few to the soup pot too (usually young roosters). To my thinking there is nothing wrong with homegrown chicken and dumplings. Since I am not a vegan, I much prefer to eat a bird I know has been humanely cared for and had a brief but happy life (they have no idea it was short) than feast on poor commercial birds that have lived in cramped, unsanitary conditions and never seen the light of day. Better food for me; better life for them.
My choices.
LofMc