Babies have a natural desire to suck. Some babies even suck their finger/thumb in the womb before they are born Thumbsucking is a common way babies seem to comfort themselves. A child usually sucks his thumb when he is tired, bored, sick, or upset or when he is not using his hands to play. A child may suck a finger(s) or fist instead of a thumb. Sometimes a security object, such as a blanket, may become part of the thumbsucking habit.
The Canadian Dental Association advises that it’s best to get your child to stop sucking before permanent teeth come in, at about age five. If a child keeps sucking a soother or thumb after the permanent teeth have come in, it could cause problems with how the jaw and teeth grow.