Unhappy news for Americans: The United States is no longer among the 20 happiest countries in the world, according to new data from Gallup and its partners.

In the newly released 2024 World Happiness Report, the U.S. dropped out of the top 20 on the list for the first time in the report's 12-year history. The U.S. now ranks at No. 23, compared to No. 15 last year. 

The researchers say this is driven in part by a decline in how Americans under 30 feel about their lives. 

"In the US, happiness or subjective wellbeing has decreased in all age groups, but especially for young adults," Gallup managing director Ilana Ron Levey told CBS News in an emailed statement, adding that social connections are one key factor contributing to these generational disparities in happiness.

"The World Happiness Report and the Gallup/Meta social connectedness data show peak loneliness for younger Americans. It's widely recognized that social support and feelings of loneliness are influential factors in determining overall happiness, and these dynamics differ across various age groups," she said. "The quality of interpersonal relationships may impact the wellbeing of younger and older individuals in distinct ways."

Finland ranked No. 1 on the overall list of the world's happiest countries for the seventh year in a row. The top 10 in the latest report are:

1. Finland
2. Denmark
3. Iceland
4. Sweden
5. Israel
6. Netherlands
7. Norway
8. Luxembourg
9. Australia

But looking more closely by age, Lithuania tops the list for people under 30, while Denmark is the world's happiest nation for those 60 and older.

"The differences in the rankings by age illustrate how people's life satisfaction ratings — which determine the rankings — vary a lot between the world's young and old." the researchers said in a news release. "In places like the U.S. and Canada, for example, rankings for those 60 and older are at least 50 places higher than for those under 30. However, in many countries, particularly those in Central and Eastern Europe, the reverse is true: The young are happier than the old."

At the bottom of the list, Afghanistan retains its spot as last in the overall ranking of happiness. 

The research team uses responses from people in more than 140 nations to rank the world's "happiest" countries, based on people's assessments of their overall satisfaction with their lives. Then to help understand the differences seen between countries, they look at six factors: the nation's healthy life expectancy, economy (GDP per capita), levels of corruption, social support, generosity and freedom.

Gallup CEO Jon Clifton said the data from the report "offers more than just national rankings; it provides analytics and advice for evidence-based planning and policymaking."

The latest findings suggest happiness has declined among 15- to 24-year-olds in North America, Western Europe, the Middle East/North Africa and South Asia since 2019, the researchers say.

"Piecing together the available data on the wellbeing of children and adolescents around the world, we documented disconcerting drops especially in North America and Western Europe. To think that, in some parts of the world, children are already experiencing the equivalent of a mid-life crisis demands immediate policy action," said Jan-Emmanuel De Neve, director of Oxford's Wellbeing Research Centre and an editor of the World Happiness Report. 

A CBS News poll in December found 21% of Americans described themselves as "very happy" and another 55% as "fairly happy." Those who said things were going well with their family lives were far more likely to report general happiness, as were people who said they have enough money to live comfortably,

The World Happiness Report — a partnership of Gallup, the Oxford Wellbeing Research Centre, the U.N. Sustainable Development Solutions Network, and the WHR's Editorial Board — was released to coincide with the International Day of Happiness, established by the United Nations, which is celebrated on March 20.