Mayor Bloomberg wants to raise to 21 the age for buying cigarettes in New York City, in hopes a higher minimum age discourages young people from smoking. He’d have done better to look at what we know about how and why youngsters start smoking in the first place.

Very few teens buy cigarettes at a retail outlet; the overwhelming majority (95 percent) get their tobacco from friends or family. So the retail accessibility of tobacco is largely irrelevant to their decision to smoke.

There’s an easy way to check this: If easier retail access to tobacco were a cause of increased smoking, then you’d expect to find less youth smoking where access to tobacco was more tightly controlled — but the real-world evidence says otherwise. The widespread restrictions on accessibility in California, for example, have largely failed to shift youth smoking levels.

Bloomberg’s focus upon first-time retail purchases also misses the essential point that most adolescent smokers experiment with single cigarettes, not packaged cigarettes. They become smokers long before they ever buy a pack. A teen’s first purchase decision isn’t about becoming a smoker, but rather about which brand to smoke.

The research evidence about adolescent smoking is full of powerful explanations for why some kids smoke and others do not. None relate to the legal age for buying cigarettes.

When asked what factors led them to start smoking, adolescents mention peer pressure, expressing their individualism and making a statement vis-á-vis their parents or other authority figures.

The seminal British government study, Why Children Start Smoking, found that the presence of any of the following factors made becoming a smoker far more likely: being a girl; having brothers or sisters who smoke; having parents who smoke; living with a lone parent, and not staying in full-time education after 16.

The most persuasive explanation of youth smoking focuses on what we know about young people’s risk factors for problem behavior. According to this “social determinants of health” approach, youth smoking is the result of lower incomes, bad schools and dysfunctional family relationships.

For example, since a study of Pennsylvania high-school students a generation ago, we’ve known that a family’s socioeconomic status is an important predictor of adolescent smoking. That is why the risk factors that lead to experimental smoking by adolescents include living in areas of high social and economic deprivation. One study found that, in an area of high deprivation, teens were 95 percent more likely to try smoking.

A number of studies have found that teen smoking is strongly linked to poor academic achievement and dissatisfaction with school. Students served by bad schools also have a 95 percent higher risk of trying cigarettes. Blue-collar parents with lower educational levels are associated with higher rates of regular smoking and higher levels of cigarette consumption among adolescents.

Family structure and functioning are also important predictors. European data shows that adolescent use of tobacco is associated with frequent parental absences and a lack of monitoring of children’s social activities.

The risks of smoking are also higher for teens not living with both biological parents, such as those living with single mothers. Young people in families with both parents present were much less likely to have tried or frequently smoked.

Siblings matter, too. US studies have found that adolescents are three times as likely to not smoke if no one in their home, including siblings, smokes.

And peer pressure: Some researchers find that a youngster’s social group and the influence of friends, particularly for girls, is the single most important factor in the decision to smoke.

The relevance of the social determinants approach led to its adoption by the World Health Organization as the basis for its tobacco control program. But Bloomberg’s unmerry band of health paternalists appears not to have noticed this critical development.

Apparently, the Bloomberg team would rather pass another unproductive ban, than actually tackle the tougher job of focusing on real solutions to youth smoking.