Millennials Surpass Baby Boomers as Largest U.S. Generation, Ending 20-Year Run

For the first time in the 21st Century, the Baby Boomers are no longer the largest generation in America.

Millennials, an age group defined as people between the ages of 23 to 38 years old, overtook their Boomer counterparts as of July 1, 2019, according to U.S. Census Bureau data compiled by the Pew Research Center and published Tuesday. Boomers, defined as Americans aged 55 to 73, had been the nation's largest living adult generation since 1999 and they held that title for two decades. Generation X, the "middle child" generation between the two, are set to surpass Boomers in population by 2028, although they peaked in size in 2015.

Pew notes the Millennial generation is continuing to grow as "young immigrants expand its ranks," while aging Boomers will continue shrinking in size as their death rate far exceeds the number of older immigrants arriving in the United States.

In 2018, Pew gathered several points of societal data ranging from labor market numbers to behavioral trends in order to offer the exact definition that the youngest 21st Century-straddling "millennials" were born in 1996. The "Baby Boomers" are defined as a generation birthed in the years following World War II, from 1946 through 1964.

Since their post-World War II inception, Boomers have always enjoyed an outsize presence compared to the generations around them. Census estimates and projections published in May 2014 show Baby Boomers peaked in size in 1999, when their population made up 78.8 million. They held that title as the largest generation until being surpassed by Millennials last year. Boomers began turning 65 in 2011.

Generation X's population peaked in 2015, with 65.6 million living adult members. During the years in which Generation X members were born, between 1965 and 1980, births only averaged about 3.4 million each year compared to 3.9 million births between 1981 and 1996 - when Millennials were being born.

By 2050, projections show the Boomer population will fall to 16.2 million. The Census projects that the U.S. population will grow from the 314 million people it contained in 2012 to 420 million in 2060. By 2050, there will be a projected 72.2 million Millennials living in the United States, driven up in numbers by migrants coming into the country.

"With immigration adding more numbers to this group than any other, the Millennial population is projected to peak in 2033, at 74.9 million. Thereafter, the oldest Millennial will be at least 52 years of age and mortality is projected to outweigh net immigration," the Pew Research Center noted in their Tuesday data sheet.

Millennials and Baby Boomers, two generations often paired together as parents and children born during the 1980s and 1990s, are not the only generational descriptors tracked by Pew. The "Silent Generation" is composed of people born between 1928 and 1945, who are now between the ages of 74 and 91. And Generation Z is the demographic term used to describe Americans born after Millennials, from 1996 through the early 2010s.

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Millennials, an age group defined as people between the ages of 23 to 38 years old, overtook their Boomer counterparts as of July 1, 2019 Micheline Pelletier / Contributor/Getty Images