Babies are born with a reflex to suck any object placed in their mouth.  This behavior helps during the first few months of establishing feeding, and disappears at around 4 months. Thumb sucking however, is not reflexive, and is believed to be a self-soothing practice.  There are usually no ill effects from thumb sucking in early childhood, and most children naturally give up the habit somewhere between 2-4 years of age.  However, if thumb sucking continues past the age of around 7 when adult teeth are coming through there can be misalignment of front teeth, or possibly exacerbation of underlying jaw ...