I have found cigarettes in my 16 yo's backpack. It really bummed me out. (I also caught him smoking weed a couple times too, which also was a huge disappointment.) I started smoking both when I was about 14 or 15, so I shouldn't have been too terribly surprised. I was honest with him about my experience with both, and I also stressed the fact that I quit and it was a HUGE effort and took TONS of willpower. I told him it's so much easier if you never even pick up a cigarette in the first place.
Just dawned on me that advice applies to the drink as well.