What do Al Pacino and Isabelle Huppert have in common? Both dominate the screen, imposing their life force on the action around them, their craft a living rebuke to the common notion that actors are like chess pieces, to be manoeuvred this way and that by the director: Pacino has his glowing, sallow death mask; Huppert her sheet metal gaze, her blood blossom mouth.

You might say that that they look “complex” – but it’s not that they are hard to read, or impenetrable, it is that their faces, repulsive and beautiful and mesmerising to others, seem to sculpt out a portion of the world all of their own. Although Huppert and Pacino work in fictional worlds, there remains something irreducible about them.

Greta Thunberg, the 16-year-old climate activist, occupies a similar register – a register that generates powerful emotions in the observer. It’s hard not to feel momentarily fixed in the spot by her eyes, framed by a face caught between stillness and expressivity. And hard not to feel immediate and frank admiration for her too – her unstudied eloquence, and her simple, resonant message: that the phenomenon of man-made climate change poses a real threat to civilised life.

It is patently obvious that we risk huge societal costs if we fail to get this right, partly because, over the past century and in the face of multiplying existential risks – conflict on a global scale, the development of the atom bomb, and natural catastrophes – nation states have proved ever more willing to take the liabilities arising from all manner of disasters onto the government balance sheet, and so, effectively, to nationalise risk. Not quite the Apocalypse – but costly? Yes.

Although she is a teenager, Greta looks like a child, and not just any child either: an archetypal Scandinavian child, part Snork Maiden, part Pippi Longstocking. There is some truth in the notion that Greta inspires such curious extremes of emotion – love, admiration, but often too, loathing and incomprehension – simply because she is a child.

Here’s cultural critic Terry Eagleton on the modern condition of childhood: “We are ready to believe all kinds of sinister things about children, since they seem like a half-alien race in our midst. Since they do not work, it is not clear what they are for. They do not have sex, though perhaps they are keeping quiet about this too. They have the uncanniness of things which resemble us in some ways but not in others.”

This sense of indeterminacy about the status of children is relatively new. In the pre-modern era, there were far fewer children around – babies faced an uphill struggle if they wanted to live past the age of four, so high was infant mortality, a trend which has skewed historical life expectancy. If you lived past your earliest years, you could expect to live a relatively long life in modern terms.

The children who did survive could expect to be employed immediately in the adult world, with most Western populations eking out an existence in subsistence farming. In Roman times, the children of elite families may not have had to work the land, but they were subjected to brutal discipline – a schooling based on enormous quantities of didactic rote learning, and moral instruction through rehearsal of the martial deeds of their ancestors. In the medieval period, young men would be sent to monastic seats of learning, or go into service with a knight where they would seek advancement through deeds on the battlefield.

But by the end of the nineteenth century, things were quite different. Indeed, Marcel Proust was able to construct an entire novel around a narrator who is a child in the first volume of his epic series In Search of Lost Time. In this way, we look into the adult world from somewhere else, entranced, along with the narrator, by Monsieur Swann’s tales of Paris high society, drawn into his obsessive desire for his Mother to give him a kiss before bedtime, his endless leisure time, which he spends reading, cloistered in his room or in the garden, or walking in the countryside. Childhood was not a route to somewhere else, backbreaking labour, or a life in service, but a time for itself, with its own rich, inner life worthy of exploration.

So what had changed? Well, just about everything, really – but perhaps it was two phenomena that created childhood in the historically peculiar state it claims today. Romanticism had given us the cult of youth, which exalted sensitivity and the nourishment of aesthetic sensibility, and rapid urbanisation had nurtured a prosperous middle class and its nuclear family, with a Father to provide for it, and a Mother to look after the children, who were under no pressure to work from a young age.

Twentieth-century culture was wracked with anxiety over the role of children – who were these semi-socialised little people? And what should we do with them? Interpretations range from their essentially demonic character on the one hand (John Wyndham’s eerie The Midwich Cuckoos comes to mind, or William Golding’s Lord of the Flies) to, in more recent times, the blandly moralistic – in the Harry Potter series and Philip Pullman’s His Dark Materials, children are repeatedly portrayed as good, essentially innocent, or instinctively creative, and sometimes all of those things at the same time.

The inverse is also true. We have an unhealthy fascination with so-called child stars, like Tiger Woods in sport, or Michael Jackson in music, who seem to be bred for one purpose only: to be a sublime talent and thereby to redeem a portion of the adult world by reflecting it in perfect miniature. In this way, we make these unfortunate children a cipher for our own hopes and fears. It obviously follows that, without having developed authentic contact with the world around them, child stars often find it difficult to adjust to the demands of adult life.

That’s what is so dangerous about the hype surrounding Greta Thunberg, within which seem to be infused all our conflicting notions of how we think children should behave. She is treated as uncanny – weird even. Or as a brilliant rejoinder to the adult miasma over climate change, daring to speak all in her innocence and her truthfulness. Or, worst of all, as a “talent”, as a star in the making. What could possibly go wrong? For Greta Thunberg’s sake, let’s hope we don’t find out.