Exposure to the fluoride ion can, in sufficient dose, induce neurotoxicity at any age, in both adults and children, but for fluoride-induced neurotoxicity to occur, in response to exposure to low doses of fluoride in the developing brain, the timing of the exposure is of importance. The evidence to date indicates that the developing brain is most sensitive to fluoride-induced neurotoxicity during the intrauterine period. Exposure to a low dose of fluoride later in childhood, at ages approximately 6–13 years, may or may not be associated with a reduction in IQ or school performance. Whether or not fluoride exposure in later childhood is associated with developmental neurotoxicity may reflect the degree to which later childhood exposure parallels intrauterine exposure. In stable societies with a single source of fluoride, such as the water supply, and no fluoride pollution from burning coal or other industrial sources, e.g., the villages of Wamiao and Xinhuai in rural China studied by Xiang et al., a higher correlation may be present between intrauterine and later childhood exposure than in societies where multiple fluoride sources are present such as industrial sources, foods high in fluoride, fluoridated salt, and fluoridated toothpaste, e.g., Mexico City studied by Bashash et al. and Thomas et al. The findings of two recent studies, in 2019, by Soto-Barreras et al. and Green et al. are consistent with this interpretation of the data.