Thus, because tobacco is addictive, the kid is inflicting a long-term, permanent harm on himself, which a child is not competent to do. Addiction is a myth. It is incompatible with free will. The 18-year old clearly has a choice to continue smoking or not. But all choices have opportunity costs, and the choice to stop smoking is no different than any other in this regard.
Smoking is undeniably cool.
Let people enjoy it when they can. Smoking is for the young. He does not engage in the nasty habit of smoking cigarettes. Obviously, he does not have any children yet.