I used to suck my thumb, stopped at 16 :( but when I decided to quit smoking I woke up sucking my thumb. Maybe I never stopped, at least in my sleep, but waking up that way reminded me how pleasurable it was so I decided to try it when I got cigarette cravings. It worked!
When people see me I just tell them I'm quitting smoking and most of them understand.
If you ever sucked your thumb, try it. At first it's a bit awkward but after a few days it felt just as comfortable as it did years ago.
Just a suggestion. If it works, don't knock it! :)