B  
Most permanent teeth (incisors, canines, premolars and 1st-2nd molars) complete enamel formation between birth and about 8 years; if water fluoride is high during this whole period, fluorosis can appear in all these teeth. Third-molar crowns usually start forming around 8–10 years and therefore escape significant exposure in a child who leaves the area at age 8 [Hegde 2016, PMID 27883981; Bårdsen 1998, PMID 10388387].