It is normal for babies to suck. That is how they relax and eat. By the time a child is two or three, he has less need to suck. If your child still likes to do so, it is better to give him a pacifier (soother) rather than let him suck his thumb.
Why? Because YOU can control when and how your child uses a pacifier, but not his thumb. Never put sugar, honey or corn syrup on a pacifier because they can cause cavities. It is best to get your child to stop sucking the pacifier BEFORE the age of three. Letting your child continue to suck his thumb or a pacifier AFTER his permanent teeth have come in could affect how his jaw grows and the position of his teeth.