In most children, thumb sucking stops on its own between the ages of two and four years. But if the habit persists after the baby teeth have erupted, it can drastically change the growth patterns of the jaw, and cause significant misalignment of the teeth. It may be hard to believe that such a benign habit can move teeth and affect bone growth, but there are a number of reasons why this occurs.
Children's jaws, especially in those under the age of 8, are growing rapidly and are relatively soft and malleable. So it really isn't hard for the constant pressure of a thumb or finger to deform the soft bone around the upper and lower front teeth. Children who are particularly vigorous thumb suckers are even more likely to change the growth patterns of the teeth and jaws.
If the thumb sucking habit persists, it can result in the upper front teeth flaring out and the lower ones moving back and inward. It can also hold back the growth of the lower jaw, while causing the upper jaw to be thrust forward. This can result in misalignment of the teeth, an anterior open bite (where the front teeth fail to close together), collapse of the upper jaw causing crossbite, or other problems. That's why it is important to stop the behavior at an appropriate time, before damage occurs.