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What is The sign of smooth surface caries in children younger than 3 years of age can be categorized as and explain in detail?
Definition 
Though the literature does not provide a universally accepted definition for early childhood caries (ECC), American Academy of Pediatric Dentistry (AAPD) defines it as "the presence of one or more decayed (non-cavitated or cavitated lesions), missing (due to caries) or filled tooth surfaces" in any primary tooth in a child 71 months of age or younger.
In children younger than 3 years of age, any sign of smooth surface caries is indicative of severe early childhood caries (7-ECC). From ages 3 through 5, one or more cavitated, missing (due to caries) or filled smooth surfaces in primary maxillary an teeth or decayed, missing, or filled score of >4 (age 3), >5 (age 4), or >6 (age 5) surfaces constitutes S-ECC. 
Reference: Pediatric Dentistry Principles and Practice SECOND EDITION, MS Muthu; pdf no 595