They do exactly that, they mix the crushed up eggshells under the chicken feed to help them recuperate. As for when they get old, they keep them until they die naturally, though they have also given some older chickens to an animal sanctuary/petting zoo thing in the region before.

I certainly abhor industrial scale animal abuse as is practiced, but as a life long vegetarian myself who is used to more harmonious animal husbandry I also believe there is an ethical way of keeping animals as livestock (and benefiting from their produce).