I suck my thumb and I'm a lot older than you. As it turns out, though, there's TONS of adult thumb suckers out there. I've found this out because, indulging publicly as I do, they've admitted it to me many times.
I've also seen adult thumb suckers many, many times over the year, all while driving.
Perhaps you should consider its many benefits. It's legal, very available, doesn't interfere with others like smoking does, effective for concentration and getting to sleep fast, calms you down instantly, doesn't ruin your health (at least for the great majority of us), and more.
So, for all you out there who think it's only for crazies, babies or what-nots, well, you probably know one yourself.