I have seen this question posed many times. My previous recollections where dust bathing started at about 2 weeks post-hatch. That in mind, I have been closely watching a hen-reared brood that my kids are helping me with as we try to train the chicks and hen for handling. The brood under observation hatched May 13, 2017. We have been releasing the hen almost daily since at roughly 1730. Observations are intense from them until they go to roost / return to pen. The hen makes a beeline to dusting area each time. She is a bit lousy as typical of hens when I overly confine them off the ground. Yesterday a couple of the chicks joined her in dust bathing for the first time. They were 5 days post-hatch at that time. Not all chicks did it. With older broods, all would do it when mother does. They are not doing it this evening owing to very heavy rain event. Earliest appears to be roughly 5 days post-hatch although not all doing so. Mother dusting seems to stimulate similar activity.