But it would also delay adulthood. Legal adulthood is the concept that after a certain point, you are responsible for your own decisions. You are free to make them, but you must also suffer the consequences of making them. Raising the threshold of adulthood keeps people in adolescence; it infantilizes them.
I don't think smoking is a good idea. I would discourage people from starting it. Seeing young people buying cigarettes discourages me. But there must come a time when people are free to do stupid things; otherwise, they are never free to do wise things.
When I was 16 or 17, we lowered the voting age to 18. 18 became the standard for adulthood -- except for drinking. I would lower the drinking age to 18, too. And I would also adjust the school entering age in order to synchronize high school graduation with turning 18. I think we would profit by a sharper, more clearly demarcated transition to adulthood. Yes, a lot of people would do dumb things. But when you delay the assumption of responsibility, a lot of people do dumb things longer, because they can, and it doesn't "count."
Not only that, but when society assumes it can delay the entry into adulthood, when it assumes that it can act in loco parentis for longer and longer, it makes its citizens into children, who must always be overseen by the nanny State. A free people must be taught about responsibility, and responsibility can only be learned by assuming responsibility.