A.C.
My little brother sucked his thumb until 1st grade. Everything we tried didn't work, plus 5 other classmates also sucked their thumbs, so he saw it as normal. My mom worked for an orthodontist, so they installed an "orthodontic appliance" (http://www.staggersorthodontics.com/whenshouldchildhaveor...) attached to his upper teeth. The applicance prevented him from sucking his thumb. Habit gone.