As a policy aimed at keeping drugs away from children, prohibition clearly doesn't work
Not even the most libertarian-minded of reformers would argue that drugs (of any kind) should be readily available to children. Likewise, one of the most common reasons for backing prohibition, especially among parents, is the belief that it will mean their children can't easily get their hands on potentially harmful substances.
As Prof. David Courtwright puts it in his book Forces of Habit, prohibition is seen as an insurance policy the costs of which are borne by people who shouldn't be behaving that way anyway.
But any government policy must be judged by its outcomes, not its intentions. And if they want to, it's not hard for minors to get hold of illegal drugs.
The UK data are confused by the figures for alcohol/tobacco and for illegal drugs coming from different surveys, by the rate for regular drinking being judged by past-week consumption as opposed to past-month for current drug use, and by 'adults' being defined as including 16- and 17-year-olds who still can't legally purchase cigarettes or booze.
Still, the numbers may yet be instructive. It's true that the actual rates of drug use are lower than the levels of drinking and smoking. But the proportions of children who drink and smoke are much lower than the proportion of adults who do, while the percentages of children and adults who use illegal drugs are around the same levels.
So perhaps it isn't significantly harder to get hold of drugs for children than it is for adults.
The US data are more damning still, despite them prosecuting their war on drugs rather more aggressively than we do in the UK.
According to data in the National Institute on Drug Abuse's High School and Youth Trends publication [PDF], the a higher proportion of children in the eighth, tenth and 12th grades reported having smoked marijuana in the past month than smoked cigarettes. (For context, eighth-graders are 13–14 years old; it's the equivalent to English year nine.)
There are a number of reasons why this might be the case. Street dealers don't ask for ID, since they're already operating illegally and don't have any licence to lose. The lower incentives for black-market trade in alcohol and cigarettes mean that children who do want them (and I'm assuming that there are more of them than those who want to use stronger drugs) have to go looking for them actively, rather than being offered – however near-mythical the idea of "pushers" actually is. And, as reported a few weeks ago, the opportunities to make money selling drugs facilitate their use.
But whatever the reasons, when more children are smoking an illegal drug than a legal one, isn't it time to admit that prohibition has been an abject failure?