Thumb-sucking and pacifier use in children younger than 4 is not considered a problem. Children usually stop this habit between the ages of 2 and 4.
Children who suck their thumbs frequently or with great intensity after the age of 4 or 5 ARE at risk for dental or speech problems. Such problems include the improper growth of the jaws, misalignment of the teeth and shape of the dental arches. A child may also develop speech problems, including mispronouncing Ts and Ds, lisping, and tongue thrusting. A custom appliance can be fabricated to help patients stop this habit. Questions regarding oral habits are common, so please do not hesitate to ask yours!
Eunice Kennedy Shriver National Institute of Child Health and Human Development