Simoun (2006)

In the world of Daikuuriku, everyone is born female, and chooses which sex they wish to become at age 17. In this world, the peaceful theocracy of Simulacrum is guarded by magical flying machines called "Simoun," which can only be piloted by young girls who haven't chosen a sex yet. When the industrialized nation of Argentum decides that it needs to invade Simulacrum to acquire the secret of the Simoun, war breaks out, drawing the Simoun Sibyllae into a lopsided battle.