http://www.ahealthybabynaturally.com/ingr/ingr320503.cfm

The American Academy of Pediatric Dentistry's 'Policy on Thumb, Finger and Pacifier Habits' says: "For most children there is no reason to worry about a sucking habit until the permanent front teeth are ready to come in."

The term "infant" derives from the Latin word in-fans, meaning "unable to speak." There is no exact definition for infancy. "Infant" is also a legal term with the meaning of minor; that is, any child under the age of legal adulthood.