Dalrymple looks at the implications of New York City’s raising the minimum age for cigarette purchases:

Such evidence as exists, however, suggests that restricting sales to minors might work. A town in Massachusetts, Needham, forbade the sale of tobacco to those under 21, and the rate of smoking among high school students declined by nearly half five years later. The rate in a neighboring town, which did not impose the ban, fell in the same time by only a third as much. Furthermore, raising the minimum drinking age to 21 was followed by (one cannot with absolute certainty say caused) a fall in alcohol consumption by adolescents, drunk driving, and motor accidents.