We've seen that prohibiting adults from a product, or vice for that matter, doesn't actually reduce health or societal damages as a result of it's prohibition. In the case of tobacco, education resulting in cultural and political awareness of the damaging effects of tobacco has done the lion's share of reducing tobacco use in general rather than restricting access. That and vice taxation as well, I reckon. Look at alcohol, age restricted sales, and use among those 18-21 is literally out of control of the education message because of a cultural effect that results in an unintended consequence. Users in that age group are likely to disregard the messages discouraging use and abuse because they consider the whole message hypocritical. I am actually in favor of reducing the age restriction on alcohol to 18 (I mean if these adults can die in the military and vote for office, surely we can expect them to be able to join the rest of adult society in imbibing a drink here and there). That cultural expectation cannot be reinforced in the current paradigm because those in that age group discard the cultural/political message about the dangers of alcohol use and abuse offhand because they are being unfairly excluded from those certain liberties.