The United States allows States, counties, cities, and towns to set their own age limits on the legal age to purchase cigarettes, so that merely crossing from one county line to another country line, or leaving one city to another city nearby, or crossing State lines, will change the legality of whether you can purchase cigarettes or not. For the most part, the purchase age for cigarettes is at age 18, but this like many laws that the "nanny state" plays with, is in the process of change and flux, so that the law essentially changes depending upon where you are physically located.
In America, you legally become an adult at age 18; although for some people through marriage they can become emancipated at an age under 18. Consequently it is the height of hypocrisy to have one set of laws for certain ages of adulthood, along with other separate laws for certain ages of adulthood, which in essence means that there are different degrees of adulthood, which logically is absurd, because either you are legally an adult with all the attendant risks and rewards or you are not. But here in America, for certain things, despite being 18 or 19 or 20 you are treated as if you are a juvenile, even though legally you are an adult for most everything else. This is incredibly bad law, a gross injustice, and indefensible.
To be an American, to be your own person, implies free will and choice, but too often in today's society, there are those that will use or create law to make crimes of things which should never be criminalized, because these certain "do-gooders" believe that they know better than you do, and consequently what you should or should not be doing. The crux of the matter is that the law should not be used as a cudgel to effect change, but instead the law should be equally applied to all, so that the setting up of special laws for adults that are at the ages of 18-20, is fundamentally a violation of the 14th Amendment which stipulates that all persons are subject to "equal protection of the laws".
For those that are so focused on the evils and dangers of smoking, do what a reasonable and fair person would do, make your argument in the public square, and devote your resources, your time, and your money to activities that support your viewpoint. To utilize instead, oppressive, unequal and unfair laws, that allow the majority to dictate to the minority, is a pox upon this country and for what it stands. Quite frankly, if those anti-smoking advocates are unable to persuade smokers to cease and desist, perhaps they need to consider that there are a multitude of factors that are involved in people's decision to begin smoking in the first place, and unless those factors and influences are replaced with something of equal interest to the smoker himself, the activity will probably continue.
There are far too many people that when they see an activity that upsets them, disturbs them, or annoys them, believe that there "ought to be a law", but alas, they should instead understand: "Or how wilt thou say to thy brother, Let me pull out the mote out of thine eye; and, behold, a beam is in thine own eye? Thou hypocrite… "(Matthew 7:4-5)