R.C.
I know people who thumbsucked for a while (myself included, to age seven) and some who've been pushed to stop. I do not believe in forcing it!
I was only doing it at night, with my blankie. When I was seven and starting braces (not BECAUSE of the thumb, because my mouth is ludicrously small), my mom told me gently that I was big and we could put the blankie in my drawer, and if I needed it, I could get it. I stopped then, easily, and it's a good memory, actually.
I know another child who has been pushed since the beginning to quit--sprays, guards, nagging, rewards, everything. Now twelve, he still does it, although he hides it quite well. And he does it more when he's been criticized! So the more the whole family nags him, the more he does it.
My daughter never even remotely wanted to do it, but my son was born doing it. He is almost three and still does it--at night or when he's really tired. He does NOT do it if he has anything better to do--playing, talking, singing, eating. So I plan to just focus on him growing up to have lots of fun interests and activities and eventually he will stop.