Babies are wired with an instinct to suck on things. It ordinarily reduces after the period of about 6 months. However, many babies will keep the habit, as it helps them self-soothe. Thumb-sucking is often observed in babies and toddlers who do it to calm down when they’re feeling hungry, scared, or restless.
In some cases, thumb-sucking in children over the age of 5 is a symptom of an emotional disorder, like anxiety.
For kids under 4, thumb-sucking is generally fine. Youngsters who do it frequently around 4 or 5, or kids who continue to suck their thumbs at age 6 could develop dental problems or speech impediments.
Continued thumb-sucking can cause the teeth to develop incorrectly aligned, or push them forward. However, this problem is commonly corrected on its own once the child has stopped thumb-sucking. But the more time the habit persists, the higher the likelihood the child will need orthodontic treatment later.
Speech problems can result also, such as lisping, pushing the tongue out when speaking, or having trouble saying the letters T and D.
For more information on thumb-sucking and how to stop this behavior, check out this website: