I was actually wondering about that too, but realized that I have had to use stuff in the past with the chicks in the coop. Meat Loaf has a red bottom himself. I’ll be cleaning the coop soon. Thank goodness Cher is so tight on the nest. I don’t think anything can survive in my nest boxes. I add a good dose of DE to those things to prevent infestations.To my knowledge there is no age limit. Untreated Lice would be more dangerous to them than exposure to the powder.
I have often treated broodies with babies. It seems broodies are very susceptible to parasites since they don't spend as much time grooming. Its not that they don't dust bath at all, but not near as much as normal laying birds.