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Typically hens slow down their egg production after a year or two of age. By that time, their meat is too tough for us to enjoy eating. Most homesteaders just keep them around, partially as pets, partially because they'll still lay eggs, but much less frequently. In commercial operations, those chickens get turned into animal feed, typically dog food.
> meat is too tough for us to enjoy eating Won't a long cooking time fix that ?
Longer cook times let connective tissue break down and is really helpful for fatty meats with lots of connective tissue.
Older chickens don't meat this criteria, they're not any more fatty than they're younger versions, and according to google, they get tough because the muscle fibers themselves get tougher, not any more numerous.