As I have mentioned previously my sister who is 24 still sucks her thumb when no one is looking. She has done this ever since she was a baby. There are lots of things you can do to stop your child sucking their thumb. Try to keep you child occupied so that they are not bored and have nothing else to do, make sure your child feels comfortable so they are less likely to need to suck their thumb for reassurance. However at the end of the day thumb sucking is not hurting anyone so why not just wait and your child will eventually grow out of this habit.