My DD sucked her thumb and by first grade hadn't stopped. The teacher told me most do when the other kids laughed at them for thumb sucking. Well she was getting laughed at and still not stopping. So off to the Dr. we went for help. First thing was to determine if it was a habit or emotional. Habits can be broken the other is much more complicated. For what it's worth we were told after evaluation testing: (condensed version) If the child doesn't care which thumb is sucked, it's a habit. If the child only sucks one thumb and won't suck the other one, it is a problem going deeper then just thumb sucking.
Our DD didn't care which thumb she sucked so it was okay to insist she break the habit If it wasn't a habit it had to be solved very differently to find the root of the problem.