It was, of all things, Love Island that started it. No middle-aged person with a pulse could sit through a single episode of this summer’s hit reality TV show, with its frantic couplings and uncouplings and endless conversations all obsessively circling around who fancied whom, and not feel at least one guilty twinge of recognition.

No, these kids couldn’t seem to think about anything but copping off with one another. And yes, that is roughly what we sounded like at that age, or would have done if remorselessly edited down for peak-time viewing. The willingness to have sex live on telly (albeit surreptitiously under cover of a duvet, or, in the case of winners Kem and Amber, an inflatable pool turtle) may be new. But the self-doubting girls and bravado-filled boys, the thrill of the chase and the sting of rejection – that hasn’t changed in for ever. All the clumsy machinations exposed in the process, the naked calculations over when to settle and when to twist for someone possibly out of your league, would have been recognisable as a mating ritual to Jane Austen (or, for that matter, David Attenborough). Millennials aren’t a foreign species, for god’s sake. They’re just the young of a very familiar one, and not even that young either.

Apologies if this makes you feel old, but senior millennials are pushing 37 now. If not exactly creaking at the knees, they’re certainly old enough to remember landlines, dating without Tinder, and annual tuition fees of only £1,000. Yet despite in some cases having children of their own they still get lumped in with kids nearly half their age in one selfie-obsessed, weirdly London-centric caricature, as if everyone under 40 was an intern living off avocado toast and communicating solely in memes.

We’ve learned not to make sweeping generalisations about men being from Mars and women from Venus, to recognise that people aren’t easily pigeonholed by gender even if some broad truths remain. But millennials and baby boomers, Generations X (middle-aged) and Z (barely out of their teens), are somehow fair game for the crudest of stereotyping.

Why are millennials held to have been invented in 1980 and abruptly discontinued in 1995? Who makes these rules anyway?

So all hail a recent report from a team of generational researchers at Ipsos Mori that challenges this idea of a vast unbridgeable chasm between people who were only ever classified arbitrarily by age in the first place. (Why are millennials commonly held to have been invented in 1980 and abruptly discontinued in 1995, exactly? Who makes these rules anyway?)

It’s true: the lives of many so-called millennials are unfolding differently to their elders, primarily thanks to their far shakier economic circumstances. They do think, and behave, measurably differently in some ways. But they also seem to think and behave surprisingly alike in others, or else are simply a few steps further down a very recognisable ideological path chosen by previous generations. (Their views on gay sex, for example, aren’t overwhelmingly different from middle-aged Generation Xers or even older baby boomers; the real dividing line is with the generation born before the war, who were well into established adulthood when homosexuality was legalised.)

Which means that much of what you read about a “millennial mindset” is dubious to say the least. They don’t actually want jaw-droppingly different things out of their working lives, the Mori report argues. And far from being lazy, if anything they put in slightly longer hours than average (although fewer than Generation X at the same age, in keeping with a decades-long trend of falling working hours). They’re not all gifted digital natives, just people with the time and patience to Google how to do stuff; and if they seem perennially glued to their phones then over-55s spend almost as many hours tethered to electronic devices, once laptops and other screens are also included.

And while it would be surprising if their expectations of life weren’t high, given their unprecedentedly high levels of education, there’s little hard evidence for the popular idea that millennials are spoilt, entitled narcissists who didn’t do enough competitive sports at school and collapse in a sobbing heap if things don’t go their way.

Employers swapping grumpy anecdotes about having to teach college leavers how to spell and put the kettle on may simply be forgetting what it was always like to be young and just starting out; soft-shelled still, with much to learn. How many Generation X-ers, if they’re honest, have mortifying stories to tell about a stupid mistake made in their first job? Let she who never wept in the office loo cast the first stone.

It’s all too easy to confuse what researchers call life-cycle effects – or things that are true of most people at certain ages, before we grow out of them – with genuine cohort effects, which are baked in and lasting. Being occasionally self-absorbed or a bit clueless is a condition of being young, not some curse unique to those born in the 80s, which helps explain why back in the 70s the generation we know now as baby boomers were themselves caricatured as a narcissistic “me generation” of kids hellbent on their own self-fulfilment.

But it cuts both ways. If not every millennial is a hysterical snowflake then equally not everyone over 55 is a selfish, reactionary leaver rolling around in piles of money, who can’t imagine why anyone would ever vote for Jeremy Corbyn. The biggest surprise of Momentum’s recent crude attempt to exploit generational differences – in a video showing a bunch of smug oldies waxing indignant over lunch about entitled young people today, while conveniently forgetting that they got their degrees for free and their houses for a song – is that all those older people who voted Labour in June, not to mention their various contemporaries currently running the party, haven’t objected more vociferously.

Time wasted arguing over which generation is definitively the worst that ever lived could be more usefully spent asking why we swallow divisive age-related myths so easily, and whose interests exactly are served by something that risks obscuring shared mutual interest. A breed of identity politics that relies on creating tribes and pitting them against one another, perhaps? Or just a marketing industry reliant on convincing retailers and employers that they alone can divine what these exotic creatures want, plus a media hungry for talking points?

Call me a snowflake, but the world hardly seems short of self-defeating conflicts and high-handed generalisations. Beware those who would turn age into another of them.

• Gaby Hinsliff is a Guardian columnist