From one of the links:
“Prior to the age of 6, you need to have some fluoride that you swallow so that it can get into the developing permanent teeth,” she said. “That’s the most important time for systemic fluoride.”
Explains why we were forced to take fluoride tablets in school. I wonder if parents (who are pro fluoride) could just do that for their children, if they can’t manage to keep fluoride in the public water supply.