The U.S. is no longer one of the 20 happiest countries. If you’re young, you probably know why.

(NBC News) Happiness is a relative concept, but an annual index that tracks it in countries around the world has found that the United States and some Western European countries are falling in overall well-being because younger people are feeling less and less happy.

The U.S., in particular, dropped out of the top 20 for the first time, falling to 23rd place from 15th last year, driven by a large drop in the well-being of Americans under 30. The age disparity is stark: The U.S. ranks in the top 10 for those over 60, but for those under 30, it ranks 62nd, pulling down the overall score.

The report tracks trends in well-being rather than causes, but one of the editors of the report told NBC News that a myriad of factors, including economic inequality between generations in the U.S., are likely to blame for the low levels of happiness in American youth.

This makes the U.S., along with a handful of other countries, such as Canada, Germany and France, the global outliers — the report found that in many regions of the world, the young are still happier than the old.

The findings, announced Wednesday to mark the United Nations’ International Day of Happiness, are part of the World Happiness Report, which has been tracking well-being ratings around the world for more than a decade. It’s based on data collected by the research company Gallup and analysis by well-being academics led by the University of Oxford in the U.K.

For the first time this year, the report gave separate rankings by age group, which in many cases vary widely from the overall happiness rankings for different nations. The report found that Lithuania topped the list for people under 30, while Denmark is the world’s happiest country for those aged 60 and older.

“We had picked up in recent years from scattered sources of data that child and youth well-being, particularly so in the United States, had seen a drop,” said Jan-Emmanuel De Neve, professor of economics and behavioral science at Oxford, who is one of the editors of the report. “That has pushed us for the first time to really slice and dice the data by these age categories, which we normally don’t do.”

The finding that in many but not all regions of the world, the young are still happier than the old, is consistent with the long-standing paradigm that people are the happiest in their younger years.

“To my surprise, youth well-being going off a cliff in the United States and North America, and to a lesser extent in Western Europe and Great Britain, is really explaining why the United States, Canada and the U.K. are getting lower and lower in the general population rankings,” De Neve said. “So that’s really explaining it because it’s not the case that the middle-aged or the people that are above 60 are dropping. If anything, the above 60s in the U.S. would be No. 10.”

Well-being for people under 30 in the U.S. ranks below the Dominican Republic, and is in line with countries such as Malaysia and Russia. Canada’s unhappy youth rank 58, four spots above the U.S.

When it comes to the tanking youth happiness in the U.S., De Neve said there is not a single smoking gun, but it is likely due to a combination of many factors ranging from political polarization to overuse of social media to uncertainty about the future and growing economic inequality between generations, with people under 30 struggling to get onto the real estate ladder.

“It’s a very complex time for youth, with lots of pressures and a lot of demands for their attention,” he added.

Meanwhile, the report also found that in countries of central and Eastern Europe, younger people are much happier than the old. But these countries have also seen the largest increases in happiness, for all ages. It was one of the biggest insights, De Neve said, that could be a big learning point.

“I think we can try and dig into why the U.S. is coming down in terms of wellbeing and mental health, but we should also try and learn from what, say, Lithuania is doing well,” he said.

The rankings are based on self-assessments by people in more than 140 countries, in which they rate their life on a scale from zero to 10, with the best possible life for them as a 10. Among the predictors of people’s happiness are not just economic well-being, the report says, but also other factors including freedom, life expectancy and social support.

This year, Finland remained on top of the list, and was followed by Denmark, Iceland and Sweden. The lowest happiness scores were registered in war-ravaged Afghanistan.

The consistently high performance of Scandinavian nations is likely down to “a high sense of contentment” and high levels of trust in the society, De Neve said.

“They are obviously wealthy nations,” he added. But more than the high gross domestic product per capita, he said, wealth is also equally distributed, “they are amongst the most equal societies, so everybody benefits from the wealth that also underpins a welfare state, which provides psychological stability.”