Age of broodiness depends on the breed and specific bird. Some hens, especially hatchery stock, never go broody b/c the trait has been bred out of them. My hatchery girls have never even show a sign of broodiness, drop the egg and run back to ranging. I have one super broody, she goes broody after about three-four weeks of laying, without fail. She "gathers her clutch" (our eggs aren't fertile, so I take hers and give her eggs to set), hatches chicks, raises them until about six weeks, and starts over. My other broody has gone broody the past two years in the middle of August. Some go broody their pullet year, and then never again. Who knows? Half the fun in having chickens is trying to figure the little stinkers out.