We are an island, populated with islands. You can see it in the way so many wept over the John Lewis advert, watching their darkest fear – to be alone, purely alone, so alone that a whole planet echoes with your trudging steps and your biggest hope of communication is a shitty telescope through which you see only a child, through a window – reflected back. And you can see it, too, in the way so many rushed to discuss it, as if the act of telling each other that they hated it or that they wept meant they were part of a community, that they had hated or wept in company rather than alone in a room with only a phone and dying peace lily to talk to. “Arrgh adverts! Arrgh trying to trick us into buying towels by reminding us we will die alone! Arrgh to all that! (Can you hear me? Guys?)”

Loneliness is very alive at the moment – we see it, facing us in Channel 4 News’ devastating film What Does it Feel Like to be Old and Alone? (“I’m always hoping that my wife will come through the door, but she never will”), and in Instagram celebrity Essena O’Neill’s big reveal (an Australian teenager went back and recaptioned her posts to show the lies behind those perfect selfies: “I took countless photos trying to look hot for Instagram… it made me feel incredibly alone.”) And in the brutal and hilarious Catastrophe, where Sharon Horgan desperately tries to connect with the one other new mother she sees as human, someone to just sit with, in a café, while the rest of the world gets on with being happy.

Like smoke or water, it gets in through the cracks in our lives when we haven’t been paying attention, loneliness, and we drown in it. Loneliness is twice as bad for older people’s health as obesity and almost as great a cause of death as poverty. But while there are structures and charities that try and eliminate its dangers for the elderly, there is very little in place that looks at the way loneliness affects the young.

As youth charities flounder, and places such as Kids Company collapse, as young people’s services are threatened with further cuts, life for many, particularly in cities, looks particularly bleak. Acevo’s ongoing Loneliness Project suggests that “London is a lonely city within a lonely nation and young people are the loneliest stratum of society.”

And make that much lonelier, I think, because there is no way to say the words. You move to a city to find yourself – a home, a job, a rest of your life. But rather than having a choice in where this life might begin, stratospheric rent and unstable work mean you find a version of yourself that must be constantly moving, sleeping in rooms that used to be stairwells, eating alone and fast. The version you share with your online networks tells a fraction of the story. The chirpy selfie, dirty washing out of shot. The single beer bottle. The grand sweep of a city lit up by fireworks, each one fizzing to its tiny death as you watch from a high-up window and gasp, less at the small explosions than the failure to catch it on camera.

There is no way for a young person to say the words: “I am lonely.” And, worse, there is nobody for them to say it to. The chief executive of the Mental Health Foundation says it can be both a cause and effect of mental health problems. There is also evidence, says Acevo’s literature review, that lonely, socially excluded young people are more likely to engage in aggressive behaviour, being drawn into gangs or extremism. It’s in everybody’s interest to prevent loneliness before the isolation starts to suffocate.

Essena O’Neill is an interesting case study, of a young person living a life online. Like the mask that eats into the face, the idealised version of herself she was projecting worked to distance her from reality. And those comparing their own unfiltered existences with those of their online acquaintances, like O’Neill’s, have been seen to withdraw socially – their glamourless flab, their eBay nights, these things can never stand up to an Instagram life.

If loneliness is killing the elderly, it is starting to infect the young. In the John Lewis ad, it’s not just the man on the moon who needs a friend – the girl, staring out into the night, is lonely, too. Except, that story wouldn’t sell half as many towels.

Email Eva at e.wiseman@observer.co.uk or follow her on Twitter @EvaWiseman