“Equal pay for equal work is hardly a controversial idea,” said Jeremy Corbyn the other day to – what else? – a youth conference for Labour activists in Birmingham, “so why are we discriminating against young people?”

A reasonable question, especially if you happen to be turning 70 this month, and trying to curry further favour with a lot of young people who already support you. To try to answer it, however, you’d need to examine the assumptions that lie behind Mr Corbyn’s statement.

For a start, there’s the implausible claim that there are any things, outside the world of pure mathematics or formal logic, that are ever equal, and the implication that it’s desirable that they should be. Next, the notion that uncontroversial ideas are by that token correct. That ignores the fact that yesterday’s controversy is today’s commonplace, an idea that, one would have thought, might have occurred to anyone whose view of the world has been shaped by the dubious Marxist doctrine of the inevitability of “progress”.

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This is what people mean when they tell you that you’re on “the wrong side of history”, as if the objective value or truth of opinions and beliefs were determined by the calendar. Oddly, the same people usually like to frame these relativistic judgments in the language of universal and eternal rights.

The most contentious bit, though, is the last part: the claim that we are “discriminating against young people”. Discriminate ought to be a neutral word that just means noting the difference between things. Nowadays, because equality is “uncontroversial”, discrimination is a Bad Thing, if not an affront to natural justice. Or, at any rate, “discriminating against young people” is. It’s apparently all right, in all sorts of other spheres, to discriminate against older people – excepting, naturally, Jeremy Corbyn.

You’ll have noted the widespread view among fanatics in both the Brexit and independence debates that the only people who voted the wrong way were old, rich and racist, and that, since they’ll be dead soon, if they aren’t already, the truly democratic result would be the opposite of what happened in the real world (which, despite the fact that it’s where history is actually happening, is presumably on the wrong side of history).

Similarly, in the climate debate, the fact that Greta Thurnberg is 16, and has encouraged children to play hooky from school to show how much they care about the planet, is in itself evidence that she’s on the right side of history. Though the spectacle of middle-aged parliamentarians queuing up to listen to Miss Thurnberg, while giving every indication that they were about to burst into the opening lines of Whitney Houston’s Greatest Love of All, is enough to make you wonder whether there is, any longer, such a thing as being grown-up.

Miss Thurnberg may nonetheless be right, of course. But assuming that young people are right because they’re young is as silly as discounting the possibility that they’re right, even though they’re young. As it happens, the current crop of people aged 16-25 look, on a good deal of the evidence, to be rather more thoughtful, diligent, sober and self-motivated than their elders – especially if you’re comparing them with the 45-65-year-old incompetents who make up most of the political and cultural establishment.

Their notions of the young are entirely inconsistent. There are constant calls to raise the age at which people can drink or smoke, to outlaw advertisements for food that includes fat, salt or sugar (ie, almost all food), restrict internet access, or to create blanket prohibitions on all kinds of adult behaviour, on the basis that such measures “protect the young”. Simultaneously, the same people argue that we should lower the voting age, pay teenagers the same as older workers and abolish any distinction made on the basis of age.

But unlike discrimination on the basis of race, sex, class or in other areas where it seems irrelevant and obviously wrong, the idea that we should not discriminate on the basis of age – or, rather, that we should do it so illogically and selectively – is bizarre.

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It was probably the creation of the generation which is now dying out, if they’re not turning their hearing aids down at Rolling Stones concerts, which only the very old and rich can afford to attend. I think that this attitude was first identified by the novelist Colin MacInnes (born 1914), who latched on to, if he didn’t quite invent, the teenager.

In Absolute Beginners (1959), he describes youth as possessing a power that adults fear and, consequently, that they keep a secret from the young. “You made us minors with your parliamentary whatsits,” the narrator tells his mother. Then “suddenly you oldos found that though we minors had no rights, we had the money power”.

Today’s teenagers and young adults can reasonably complain that that generation – the “My Generation” of perpetually adolescent contemporaries of Mr Corbyn – has hung on to “the money power” while denying them the same opportunities. The problem with these old people, in fact, is that they’re too young. But there remains a case for treating young people differently – and not merely the economic ones that a higher minimum wage might reduce employment opportunities from that age group, or that the state has no business dictating the value of labour.

We discriminate against young people because they are in some respects adults, and in others, not yet fully responsible. There are many aspects of the world in which youth has the whip hand over age – the greatest achievements in poetry, mathematics and popular music, for example, have tended to come from the very young. There are others in which the qualities of youth – idealism, urgency, energy, the infinite sense of possibility, reluctance to compromise, impatience with restraint, refusal to distinguish between one’s passions and their intrinsic value - create hazards as great as their advantages.

That may be a depressing-sounding argument for age before beauty, cynicism over idealism, indifference over enthusiasm, and hoping to grow old before you die. But the old know what it’s like to be young, while the young can’t know what it’s like to be old. In matters of age, there’s a case for discrimination, which used to be a praise word synonymous with wisdom, judgment and, above all, maturity. Mr Corbyn wouldn’t understand that, but then he’s not a grown-up.

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