It might not have a thing to do with age but more with personality type - the obsessive/compulsive type. I was in a class with adults last summer and some of them couldn't stop either. Our instructor always asked us to find a stopping place and gave us a chance to finish the current thought. Then she would say, "You might have to cover the cubes with a piece of paper so you can give me your attention." I ask my students to put their hands in their laps or behind their backs. A "look at me and smile when you are ready" works sometimes. I think you need to respect their need to finish their thought but also hold them to giving you complete attention when it's discussion time.
