From the article > Fluoride can readily cross the placenta, accumulate in the infant brain and easily exert neurotoxic effects, such as decreasing norepinephri

From the article > Fluoride can readily cross the placenta, accumulate in the infant brain and easily exert neurotoxic effects, such as decreasing norepinephrine in the parietal and occipital lobes, decreasing serotonin in the parietal lobe and increasing serotonin in the frontal and occipital lobes [42-45]. So maybe it passes the blood-brain barrier more easily in infants or in utero? Also if you search for "rats' in the original article, http://www.ehjournal.net/content/pdf/s12940-015-0003-1.pdf http://www.ehjournal.net/content/pdf/s12940-015-0003-1.pdf then you'll see them referencing studies that has rats drink or be injected with fluoride. They weren't referencing the study you linked.