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farmtransparency.org/…/age-animals-slaughtered

One factor here is that they typically kill egg laying hens a little bit later than they do for broiler chickens. They do so once they are 18 months old since egg laying rates declines and they are then seen as waste. This is in comparison to broiler chickens who are slaughtered even younger at just a couple of weeks old. Faster to bring back the broiler population after killing all of them due to disease if you were going to kill them in a few weeks regardless of disease