Editor's note: Laurence Steinberg is professor of psychology at Temple University and the author of "You and Your Adolescent: The Essential Guide for Ages 10 to 25."

(CNN) -- The steady decline in teen smoking has stalled.

According to "Monitoring the Future," the University of Michigan's annual survey of the nation's adolescents, in 2006, about one-fifth of U.S. high school seniors smoked at least monthly, down from more than a third who did so in 1996. But in the past few years, that figure has not continued its steady drop.

The decline in regular smoking among younger students has similarly slowed, with rates at about 7% for eighth-graders and 14% for 10th-graders. Indeed, signs in the most recent surveys indicate that smoking among younger teenagers might be on the rise.

No one knows why rates of teen smoking haven't continued to fall, or what to do to get them to start dropping again. Virtually all American students still receive some sort of anti-smoking education, and there has been no weakening in restrictions on tobacco advertising.

The cost of cigarettes, usually a strong predictor of a decline in teen smoking, continues to rise. Yet more than 40% of today's high school students have tried smoking before they graduate, according to the Centers for Disease Control and Prevention.

Perhaps anti-smoking education has done about as much as it can be realistically expected to accomplish. We should not discontinue it, of course. But we need some other strategies. At the top of the list should be raising the minimum legal age for purchasing tobacco to 21.

Smoking is by far the leading preventable cause of death in the United States. Because almost every adult smoker began smoking during adolescence, preventing teenagers from trying cigarettes is critical.

There now is solid evidence that smoking during early and mid-adolescence is far more likely to lead to addiction than is the same degree of smoking after age 21. (Animal studies similarly find that exposure to nicotine shortly after puberty is more addicting than exposure once adult maturity has been reached.) This is because the brain systems that are active when we experience pleasure are highly malleable during adolescence, and far more easily modified -- sometimes permanently -- by exposure to all sorts of drugs.

After 21, the same brain systems are much harder to change, which is good news for those who haven't tried smoking, drinking, or illicit drugs, but bad news for those who by that age are already hooked.

Some public health experts have called for graphic pictorial warnings on cigarette packs -- pictures of diseased lungs, people having heart attacks, corpses, and the like -- and late last year the FDA unveiled the warning labels it will require beginning in 2012.

A few studies in countries where such warnings are already required have found that these labels may help motivate adult smokers who already want to quit. But there is no evidence that they will prevent teenagers from taking up the habit, and there are many reasons to think that they will not work. American teenagers well understand the risks of smoking and know that it has harmful long-term health consequences.

The problem is that many adolescents believe that it is possible to smoke occasionally without becoming addicted -- yet more than a third of individuals who start smoking as teenagers become daily smokers by the time they turn 18.

Reminding adolescents that cigarette smoking is potentially addictive, as some of the pictorial warnings will do, sounds like a good idea, but at least one study of adolescents' understanding of addiction cautions that telling them that smoking can be addictive may make cigarettes more, not less, attractive. When teenagers hear that something is addictive, they take that as an indication that it must be enormously pleasurable.

An alternative strategy to educating adolescents about the risks of smoking is limiting their access to cigarettes. This is an enormous challenge, though, because many adolescents who smoke don't get their cigarettes from retail outlets. They get them by bumming them from their friends. In order to limit teenagers' access to cigarettes, we need to find a way to get them out of their social networks.

The surest way to accomplish this is by raising the minimum legal purchase age for tobacco to 21. Currently, most states set this age at 18. (A small handful, as well as a few counties in New York, set it at 19.) The problem with the current arrangement is that it virtually guarantees that cigarettes will make their way into the hands of younger teenagers, with whom 18-year-olds regularly socialize.

Restricting sales of cigarettes to individuals 21 and older will both limit their legal sale to individuals who are less likely to become addicted, and will keep more cigarettes out of the hands of younger individuals who are far more vulnerable. According to one estimate, raising the purchase age to 21 would cut the proportion of high school students who smoke to less than 10% within seven years.

That, indeed, would be a breath of fresh air.

The opinions expressed in this commentary are solely those of Laurence Steinberg.