But any long-term harm incurred will be due to his decisions, as an adult, to continue smoking. It will not be because of a few years of youthful fun. Thus, the kid's smoking does not pose a long-term danger to his health. The newly-minted smoking adult cannot simply choose to quit smoking, because of the addiction inflicted on this body when he was a minor. 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.