My DS does this, he is almost 5 and about to start school. The v annoying thing about it is that he started when he was 3 as his older cousin is a sucker so he copied him!
So he is now an avid sucker himself, which although I dislike, it is combined with hair twiddling, he twists his hair round and round at the crown of his head and pulls it out. This means he has a little bald patch. This I hate!
So we are thinking of buying those plastic thumb guards, which at an eye watering £55, plus postage! But the testimonials on the site are convincing. seems pretty good
Have you used it? Does it work? TIA