As it happens, this is a good argument for cannabis: that it has some medicinal application. As does heroin — based on opium, it remains the greatest of all painkillers. But I would also suggest that the question of cannabis presents a strong case for age discrimination.

The medical evidence seems to be that cannabis, taken in moderation, does not necessarily have a destructive impact on older people. It might make them a little senile, a little earlier, if they overdo it, but it is unlikely to cause them to harm themselves or society by developing schizophrenia.

The health dangers of cannabis are much greater for the young, particularly those under 18 — or worse, under 16 — as the researches of Professor Robin Murray of the Institute of Psychiatry in London have indicated.

The brains of teenagers are still under construction, and are likely to be much more vulnerable to mind-altering substances. Certain young people, vulnerable to schizophrenic episodes, may have such schizophrenia (or other mental disorders) triggered by marijuana.

Alarmingly, some psychiatrists are saying that we are facing a serious epidemic of mental illness because very young people are using cannabis, and it is disturbing their brains. The age at which cannabis is first smoked (or ingested) is the crucial factor in subsequent psychosis triggered by the drug.

It is difficult to make a law that proclaims that something is acceptable at 66 but is not acceptable at 16, although we do have alcohol regulations which depend upon age qualifications. But as a matter of health education, it should be made known that a cannabis cookie baked by your hippy grandma is not half as harmful, if eaten by her peers, as it would be if taken by vulnerable teenagers whose own brains are not yet finished being baked.

Here, at last, is an authentic case for oldies saying “do as I say, not as I do”.