It's what babies do to soothe themselves and they usually stop on their own by 3-5 yrs old but some kids keep on with it to adulthood and in your girls case, her parents didn't put a stop to it. My mom had to use mitts and bittering agents to my brother's hand every night to make him stop.
Folks with buck teeth or crazy overbites most likely warped their shyt through years of thumbsucking.
Folks with buck teeth or crazy overbites most likely warped their shyt through years of thumbsucking.