Oral health risk assessments should begin at six months of age, just before the first tooth erupts.
Many clinicians find that, especially when starting to perform risk assessments, using a formal tool aids in oral health risk determination, targeting of anticipatory guidance, and in documentation.
Fluoride varnish is recommended for all children ages 5 and under, independent of caries risk. A risk assessment tool may help you decide which children would benefit from more frequent varnish application, as the American Academy of Pediatrics 2014 guidelines recommend fluoride varnish twice annually for all children and 4 times annually (quarterly) for children at high-risk for caries.
American Academy of Pediatric Dentistry: Council on Clinical Affairs. Guideline on Caries-risk assessment and Management for Infants, Children, and Adolescents. Revised 2013. REFERENCE MANUAL V 35 (6): 124-130.