Dental fluorosis is caused by taking in too much fluoride over a long period when the teeth are forming under the gums. Only children aged 8 years and younger are at risk because this is when permanent teeth are developing under the gums.The severity of the condition depends on the dose (how much), duration (how long), and timing (when consumed) of fluoride intake.nly children aged 8 years and younger can develop dental fluorosis because this is when permanent teeth are developing under the gums. 1  Once the teeth erupt through the gums and are in the mouth, they can no longer develop fluorosis.