Commitment Hour by James Alan Gardner

Commitment Hour is a science fiction novel by James Alan Gardner, published in 1998. The novel is set in Gardner's "League of Peoples's" futuristic universe, and plays out in the small, isolated village of Tober Cove. Set on post-apocalyptic Earth, Tober Cove most resembles a rural, seventeenth century fishing village, with one exception: every year, everyone below the age of 21 changes gender.