J.J.
My daughter sucked her thumb and we got her to stop when she was about 3 or so. The dentist gave us 3 months or he told her he'd have to put an appliance in her mouth. We got a book called "Harold's Hideway Thumb". We had her help think of ways to keep her thumb out of her mouth - put a string/ribbon on it, a glove, etc. Nighttime was the hardest, since she didn't realize she was doing it. We also got some liquid to put on the thumb, I think it was called "Thumb", ask a pharmacist if you can't find it. We told her how awful it was - and she was too afraid of what it might taste like that she didn't put her thumb in her mouth. It did take about 3 months. She also had a stuffed animal that we let her keep. Maybe let her have the stuffed animal- just put a glove on her hand, or ace bandage on the elbow like someone else mentioned. Good luck.