i can absolutely see your point bekissed. I think we have a responsibility to care for the flock, sometimes that means a cull, i have heard that older chicken isnt that great to eat though, a little like mutton, tough unless cooked for a very long time.
Not sure i fully see eye to eye on the age thing though, i am sure some chickens, if well fed, sheltered and protected from predators will live to an old age, and die, of natural causes like every other animal.
I have a beagle, he goes everywhere with me, apart from the wife, he is next in command, if he becomes sick, he will get put down at the vet, apart from sickness or disease, he will die, in his sleep, in his kennel, I wont "pre empt" his possible suffering by euthanizing him first, i have the same feeling with my chickens, if they can live to an old age, doing what they love, i will leave them be, if they are sick or suffering, i will put them out of their misery