My ten year old is exactly the same, and always has been. She's always chewing on *something*. Interestingly, she also has trouble eating enough to keep weight on, and doesn't particularly like eating. She has some kind of sensory seeking behaviour, but is then overwhelmed if it tastes of something! She'll only eat the blandest of foods.

The best thing I can think of is to find them something they can safely chew on, and to help them develop some stress maintenance technique for when it is inappropriate to put things in their mouths. My dd for instance has taken up drum corps this year and her fidgeting seems to have transferred from sticking stuff in her mouth to tapping out rhythms on the table... equally aggravating LOL but easier on her teeth!