Thumb sucking at an early age is considered to be a normal and natural activity among children. It grows out of very strong sucking reflex, which is vital for child"s survival. While babies use sucking in order to get nutrition, they experience certain pleasurable oral sensations, some of which associated with the fulfillment from food, closeness, and tenderness. In most cases, as the child matures he or she finds alternative means of receiving the same forms of positive emotional fulfillment.
However, some children look for a way to continue the once necessary and pleasantly soothing experience of nutritional sucking (either breast feeding or sucking from the bottle), thus forming the thumb (or finger) sucking habit. This habit, if not eliminated by a certain age, may become detrimental to a child"s physical, emotional and social development. Recent scientific studies and guidelines from a major dental institution indicate that children should stop thumbsucking between ages of three and four as opposed to the "rule of thumb" approach used by some pediatric dentists; stating that the habit should be eliminated before the arrival of permanent teeth (ages five to six). & 
Another recent study suggests that thumb and finger sucking should be eliminated before the "terminal plane" is damaged. "The results of this study suggest that thumb and finger sucking should be eliminated before damage is done to the terminal plane. This would appear to be between three and four years of age.
Vigorous thumb sucking may profoundly alter the development of the mouth and the positioning of the teeth in the upper and lower jaws. As a result, the roof of the mouth is pushed upward and narrows, which leads to the development of a cross bite. Improper positioning of the front teeth and tongue can often lead to speech impediments (the inability to pronounce certain sounds). Rather expensive orthodontic work, involving lots of time, effort, and unpleasant dental procedures would be required to correct these problems (if the habit is not eliminated at the right time.)
Mentioned here are other problems associated with thumb (finger) sucking:
an increased risk of communicable childhood diseases, as a result of constantly putting a dirty thumb in the mouth;
a possibility of slowing down child"s social development because children who suck their thumbs (fingers) are perceived by their peers as babies, and in many instances, either not accepted by the group as equal or ridiculed by their peers.
In order to relieve stress, due to competitive encounter with other children or with an unpleasant situation, the child would choose thumb sucking, in a desire to withdraw, instead of solving the problem.
It"s important to remember that if thumb sucking is extinguished at an early age, many of the problems listed above may naturally fix themselves, including the shape of the jaws and positioning of the teeth. A study suggests that "natural correction of malocclusion caused by thumb and finger sucking is related not only to the degree of malocclusion, but also to lip and tongue function as well as that of other perioral musculature."
1. "Damage to the primary dentition resulting from thumb and finger (digit) sucking", Journal of Dentistry for Children. Nov-Dec 1996.
2. "Influence of thumb sucking on peer social acceptance in first-grade children", Pediatrics. April, 1994.
3. American Academy of Cosmetic Dentistry. "Thumbsucking - stop it early." Feb. 1, 1999.