Thumb sucking is common for children till the age of 3-4 yrs. How ever it will become harmful if continued beyond this age.
Thumb sucking will exert an outwardpulling force on the jaw.
At 4 yrs thepermanent teeth start erupting in to mouth with downward force like a landing aeroplane.The force of erupting teeth and the dragging force of the sucking thumb or finger will lead to forward displacement of the erupting teeth and bone also. Hence immediate attention should be paid if this sucking continued after 4yrs, to prevent the coming out of the teeth.
The urge for the sucking of the thumb is as a result of closer adaptation of the finger or thumb to palate. The break in this adaptation will make the child stop this habit immediately. An Orthodontist can well plan for this depending on the condition.