Yes, the correct response is 6 months, but that answer needs to be qualified.

Observe the chart below to see that after a family's water has been tested (contact your State Health Department or your local water provider) to determine its level of fluoride, the clinician needs to then determine the age to begin supplementation as well as the amount to supplement. So, a 6-month old baby that is consuming water that is less than 0.3 ppm (parts per million) of fluoride would need a 0.25 mg supplement.

Systemic Fluoride Supplements: Recommended Dosage Schedule of the
American Dental Association, the American Academy of Pediatric Dentistry
and the American Academy of Pediatrics

Age	Fluoride ion level in drinking water (ppm)*          < 0.3 ppm                       0.3 - 0.6 ppm              > 0.6 ppm
Birth - 6 mo.	None	None	None
6 mo. - 3 yrs.	0.25 mg/day	None	None
3 yrs. - 6 yrs.	0.50 mg/day	0.25 mg/day	None
6 yrs - 16 yrs.	1.0 mg/day	0.50 mg/day	None

*1.0 ppm = 1 mg/liter