You’re Not Alone
Thumb sucking is one of the most common habits during childhood. Most children suck their thumb for safety even before birth. Roughly one out of every three children, ages 1 to 4, will suck his or her thumb at least sometimes. About one in five children will still be doing so at the age of 5 or older. The habit is typically harmless if the child does it occasionally, but thumb sucking is one of the most difficult habits to break.
Why It Happens?
According to the American Dental Association, Children suck on things because sucking is one of a baby’s natural reflexes and as infants get older it serves them many purposes. It may make them feel secure and happy and it also helps them learn about their world by sucking on their fingers, thumbs and other objects. Young children may also suck to soothe themselves. Since thumb sucking is relaxing, it may help induce sleep.
Children should have stopped sucking their thumbs by the time the permanent front teeth are ready to erupt. Most children usually stop thumb sucking between the ages of 2 and 4.
About fifteen percent of children will continue thumb sucking past their fifth birthday. This is an age when teasing often starts, causing social difficulties for children once they reach school age.
What Causes Thumb Sucking In An Older Child?
Some children are more nervous, shy , or insecure than others, and may use thumb sucking to relieve tension. Others may resort to thumb sucking to alleviate boredom or maybe when they’re tired or upset. They may also use his thumb to fall asleep at bedtime and to lull themselves back to slumber when woken up in the middle of the night. Sometimes parents unwittingly prolong the habit by getting into power struggles with their child in an attempt to stop thumb sucking.
The good news is that while thumb sucking is common in toddlers and younger children (about half of all children do it at some time or another), it tends to disappear by age two or three, except in those few children who become fixated. Nelson's Textbook of Pediatrics gives the incidence by age 5 of 10% of children.

When Should I Suspect My Child Has A Thumb Sucking Problem?

For the child under two, thumb sucking is rarely, if ever, considered a problem, and it may help to alleviate the child's stress or tension. But older children who suck their thumb or finger chronically may need guidance from parents because the chronic sucking habit can cause the child's permanent teeth to become crooked.
"The kids you worry about are the ones who suck their fingers while watching TV and while they're in the car and while they sleep and oftentimes, during school. These children can develop dental problems," one dentist states.
Now is the time to deal with the problem, which if persistent, can interfere with tooth eruption and position, create speech issues, affect the growth and development of jaws, and perpetuate abnormal swallow patterns.