C.A.
I just asked this question myself not too long ago, and got some interesting responses. I'm glad you asked too because I now have more ideas.
My son is 7 as well, going into the second grade, and he sucks his thumb too. I think I have to agree that it is really hard (not quite impossible, though I think detrimental) to force them to stop if they're not ready. When he is busy/distracted, he tends to suck his thumb less, but it can be hard to try to keep a child busy like that 24/7 (if for no other reason than it's a drain on your sanity to try to keep your attention on a child for that long ;)).
Also, I realize that really I know a bit of how it is from personal experience as well. Not with thumb-sucking, but with picking at my skin and/or plucking hairs. It's an obsessive thing, really, and tends to be worst when I'm anxious, or bored. Same with me, if I'm distracted, I do it less, but keeping distracted enough to keep from doing it is hard. Plus, sometimes the harder I try -not- to, the worse it gets (kind of like how trying too hard to get to sleep makes it harder to sleep).
I also suspect that part of the reason I have such a problem with it is because I had the thumb-sucking "beat" out of me...That makes it hard for me as well dealing with my son. I don't want to push him too hard, but sometimes seeing him suck his thumb all the time drives me nuts...
Trying to limit the behavior to certain rooms and/or times sounds fairly reasonable to me, I might have to try that...then he can shove his hands down his pants at the same time as he is so fond of doing...*sigh* ;)