You definitely want to get on top of this NOW!! I personally didn't have any problems with breaking my daughter of the habit, and my son never did it. So I guess I've been blessed. But others haven't been...
My sister's son still sucks his thumb... HE'S ALMOST 14!!!! He hides it really good though. But last month we threw his dad a surprise 40th birthday party. The cake that was there had a dye in the frosting that turned everyone's lips, teeth and tongue a very dark blue. During the party, my mom noticed my nephew... not only was this mouth blue, but so was his THUMB!! LOL
My husband's niece, who's now in her mid-to-late 20's, we suspect, is still sucking her thumb. So I think it's something you really need to work on.
My sister contributes her 14 year-old's bad habit to a power-struggle when he was a toddler. He's still very stubborn-headed. When he was about 2 1/2, he was told by one of his uncles NOT to suck his thumb, when he wasn't even doing it. My sister says that since then, it's become a major thing for their family. They tried ignoring it, putting something on his thumb, punishing him, and telling him about how it can effect his teeth... NOTHING HAS HELPED!
So they got to thinking it's probably going to have to come from peer-pressure to break him of it. Only time will tell.
Sorry I haven't been of much help... just wanted to tell you the sooner, the better! ls