UN-commissioned study in UK, US, Ireland, Australia, Canada and New Zealand finds a mental health crisis that means “youth is no longer one of the happiest times of life”.

In a research paper commissioned by the UN, leading academics Jean Twenge and David Blanchflower warn that a burgeoning youth mental health crisis in six English-speaking countries worldwide is upending the traditional pattern of happiness across our lifetimes.

Whereas happiness was once considered to follow a U-shape – with a relatively carefree youth, a tougher middle age and a more comfortable later life – wellbeing experts now suggest life satisfaction rises steadily with age instead.

“The U-shape in wellbeing by age that used to exist in these countries is now gone, replaced by a crisis in wellbeing among the young,” according to the paper published by the US National Bureau of Economic Research.

The study found that life satisfaction and happiness has fallen among young people over the past decade, and particularly among young women.

It highlighted the rise of smartphones and social media, suggesting the trend coincided with the growth of internet usage.

“The young have become isolated. It’s also not so much that people are sitting there on the phone, it’s what they’re not doing. They’re not going out as much; playing with their friends, interacting with others, or having as much sex,” Blanchflower, a former Bank of England policymaker told The Guardian.

“I don’t think there is any doubt you have an absolute global crisis. Young people are in deep disarray and trouble.”