All babies are born with a sucking reflex. They will suck on a nipple, thumb, fist, finger, or anything else that comes in contact with their mouths.
Babies soon come to associate sucking with feelings of security.
As they grow older, it's normal to suck on thumbs and fingers. The habit soothes and calms the child. About 9 out of 10 children engage in the habit during their early life. Over half of thumb or finger suckers stop by age 6 or 7 months. Most will have stopped by age 4 years.