A woman takes a cigarette break in Wise, Va. (Paul J. Richards/AFP/Getty Images)

FORTY-TWO million Americans still smoke. That is a much smaller proportion of the population than decades ago. But it’s still a public health disaster: Eighteen percent of adults put themselves and their families at risk of major and wholly preventable health problems. Education programs, changing social attitudes and higher tobacco taxes have pushed the smoking rate down, and cigarette bans have made the air a lot less foul in public places. But a new report articulates the logic behind an additional approach to fighting tobacco: Raise the age at which people can legally buy tobacco products to 19, 21 or even 25. Cities, states and even Congress should consider this option seriously.

Most states require tobacco buyers to be at least 18. But four states have moved the age to 19, and New York City and a few others have raised it to 21. Congress didn’t go so far in its overhaul of federal tobacco regulation several years ago. But it did order a report on the impact of such policies. The Institute of Medicine found that increasing the age to 21 would reduce smoking by 12 percent. It found much smaller effects for an age limit of 19 and slightly better results for 25.

These findings make sense. Nicotine’s pleasurable effects are strongest on teenage brains, making continued smoking and eventual addiction much more likely if experimentation begins early. It’s no wonder that 85 percent of daily smokers started before age 21. Underage smokers by and large get their tobacco products, primarily cigarettes, from friends and acquaintances. Part of the rationale for increasing the legal age to buy tobacco is to isolate adolescents from social connections that enable them to light up. Another part is to further adjust social norms and expectations about teenage smoking, particularly in the way parents and schools treat the issue.

The public health benefits of the projected decline in smoking from raising the age to 21 would take years to realize, but they would be significant. There would be 249,000 fewer premature deaths, 45,000 fewer lung cancer deaths and 4.2 million fewer total years of life lost among those born between 2000 and 2019. Not to mention how much healthier people would be in the years they are alive. The benefits wouldn’t end with smokers themselves: Between now and 2100, 286,000 fewer babies would be born prematurely , and the effects of secondhand smoke on children would diminish. The researchers insist that their projections are conservative.

It’s tricky figuring out when to treat young people as adults — mature enough to drink, smoke, vote, fight or engage in other activities. Society hasn’t settled on a single answer, sending a contradictory and undoubtedly frustrating message to those on the cusp of adulthood. But, the report shows, while policymakers aren’t about to clear up that mixed message, 18 isn’t the soundest place to set the smoking age. Cities, states and the federal government should listen.