When are kids (teenagers) too old to have an open nudity in a house of mixed genders?

In my husband's family, nudity was freely practiced. The older brother, in highschool, would be "airing himself (their words)" infront of his slightly younger sisters and mother. It was a mandate that you had to leave your towel in the bathroom and walk naked to the bedroom or take a bathrobe (which the boys/father did not opt for). The siblings still have this liberalism with their own children, now, who are in elementary and middle school. Am I just a prude or do kids need some type of privacy?