It normally stops before the appearance of the second teeth. It disappears in fits and starts. It decreases rapidly, but returns when the child feels the need for some additional comfort. The good news is that eventually it will go for good. It is too much to hope that the child will kick the habit before she is three.
Children usually outgrow thumb-sucking between the ages of three and six.
If your child occasionally sucks her thumb, but generally seems happy and well-adjusted, there is no cause for concern. However, thumb-sucking can be an indication of maladjustment or lack of love. Parents should try to identify what is bothering the child and then set it right if possible. May be your child needs companionship, or may be you are being too restrictive or not providing enough stimulation and distraction. There could be any number of reasons.