In order to study the age-related susceptibility to dental fluorosis, 40 children who had been lifelong consumers of moderate- to high-fluoride water (0.55-8.48 mg F/l) were examined, as well as a group of older siblings (n = 40) who were born 6 months to 6 years before the fluoride-containing drinking water was introduced to the household. Background information was obtained through a structured questionnaire. Dental fluorosis was scored according to the TF index. Among the 80 children examined, the permanent incisors were erupted in 66, while 67 had permanent first molars present. As compared to their older siblings, the prevalence of dental fluorosis was significantly higher in the children who had consumed moderate-to high-fluoride water throughout their lives. In a multiple regression analysis, the variable “age when introduced to moderate- to high-fluoride water” came out as the only significant risk factor associated with dental fluorosis. This variable was divided into three categories according to the first exposure to moderate- to high-fluoride drinking water (1) 0-12 months of age, (2) 13-24 months of age and (3) after 24 months of age. Category 3 was used as the reference group. Fluoride exposure starting during the 1st year of life showed the highest odds ratio as compared to exposure only after 2 years of age. The findings indicate that early mineralizing teeth (central incisors and first molars) are highly susceptible to dental fluorosis if exposed to fluoride from the first and–to a lesser extent–also from the 2nd year of life.