Utter nonsense. If that's true, then how would chicks with a broody hen survive? She takes them with her off the nest when they are only a few days old. She sure isn't waiting until they are adolescents to bring them outside (and she likely would have weened them long before that point). They absolutely do have an immune system well before then, obviously, the area should be kept reasonably clean, but they definitely don't need a sterile environment. I usually have them outside full time by 5 weeks but do trips outside well before then and I give them clumps of grass with the dirt when they are just days old. Never had an issue