As recreational drugs go, marijuana is relatively benign. Unlike alcohol, it doesn't stimulate violence or destroy livers. Unlike tobacco, it doesn't cause lung cancer and heart disease. The worst you can say is that it produces intense, unreasoning panic. Not in users, but in critics.

Those critics have less influence all the time. Some 18 states permit medical use of marijuana, and in November, Colorado and Washington voted to allow recreational use. Nationally, support for legalization is steadily rising. A decade ago, one of every three Americans favored the idea. Today, nearly half do—and among those under 50, a large majority does.

These trends have diehard drug warriors screaming bloody murder. Former Rep. Patrick Kennedy, D-R.I., has formed a new organization to stop what he imagines to be the "300-miles-per-hour freight train to legalization." He says that such a change would be especially harmful to teenagers.

White House drug czar Gil Kerlikowske insists that even allowing medicinal pot "sends a terrible message" to adolescents. Mitchell Rosenthal, a psychiatrist who founded the substance-abuse treatment group Phoenix House, says there is "mounting evidence of the dangers it poses, especially to young users."

They might have a point if existing drug laws were keeping weed out of the hands of wayward kids. In truth, they're about as effective as a picket fence in a tidal wave. In a 2009 survey, high school students said they found it easier to get than beer. In 2011, 23 percent of 12th-graders said they had used weed in the preceding month.

In the past five years, drinking and cigarette smoking have dropped by more than 10 percent among high school seniors. But pot smoking has risen by 23 percent. Alcohol and tobacco are legal for adults. Marijuana is not.

What these trends indicate is that authorizing the sale and use of a substance does not necessarily mean more people will use it. There is no contradiction between letting adults make up their own minds, with some government regulation, and providing effective education for youngsters about the hazards of underage consumption.

No one, after all, is talking about putting pot in vending machines or handing out blunts at Taylor Swift concerts. The idea is to treat pot like booze—permitting its sale and use to adults in a government-regulated market, with penalties for behavior (like driving under the influence) that endangers other people.

The tolerance-fuels-use theory is thunderously lacking in real-world support. In the Netherlands, where "coffee shops" are allowed to sell pot, teenagers are far less likely to use it than their American peers.

The experience here falls short of bloodcurdling. "In the states that have passed medical-marijuana laws, youth marijuana use has decreased," Amanda Reiman, policy manager for the Drug Policy Alliance, told me. In California, "the number of 7th, 9th and 11th graders reporting marijuana use in the last six months and in their lifetimes all declined" after 1996, when the state passed its medical marijuana law.

The alleged harms of cannabis on the teen mind and body are generally exaggerated. Critics have trumpeted a study last year that said teenagers with a heavy habit turn out to have lower IQs as adults than their peers who avoided the stuff.

But a new assessment by Norwegian scientist Ole Rogeberg, published in the Proceedings of the National Academy of Sciences, concluded that the IQ differences might well stem from differences in income, education and other factors. "The true effect," he said, "could be zero." It's pretty clear that heavy drinking is a far bigger danger to developing brains.

Those worried about the welfare of potheads might also want to take into account the dangers that exist only because cannabis is illegal. Criminals who grow or supply the stuff have little incentive to monitor quality, prevent adulteration or assure consistent doses.

A kid who gets his hands on beer doesn't have to worry about getting toxic chemicals or nasty fillers. Buying pot in illicit markets may also expose users of all ages to violence, robbery or extortion. But you don't see innocent bystanders getting killed in shootouts among liquor store owners.

The alternative to legalization is sticking with a policy that has produced millions of arrests, squandered hundreds of billions of dollars and turned many harmless people into criminals in the eyes of the law—all while failing to stem the popularity of pot. For kids or adults, there is nothing healthy in that.