It is normal for babies to suck. That is how they relax and eat. By the time children are two or three, they have less need to suck. If your child still likes to do so, it is better to give them a pacifier (soother) rather than let them suck their thumb.
Because YOU can control when and how your child uses a pacifier, but not their 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 children continue to suck their thumb or a pacifier AFTER their permanent teeth have come in could affect how their jaw grows and the position of their teeth.