Millennials overtake baby boomers to become nation's largest generation

Millennials, so called because the generation's birth dates straddle the year 2000, are defined as those who were between 18 and 34 years old in 2015. The baby boom generation, which got its name from the spike in births following World War II, were between 51 and 69 years old that same year.

The growth in the nation's millennial population has been aided by immigration, which has added more numbers to the generation than any other. The population is projected to peak in 2036, with 81.1 million millennials, before mortality rates are expected to outweigh immigration gains, according to estimates from the Pew Research Center. In that year, the oldest millennial will be 56 years old.

By 2050, researchers estimate the generation will include 79.2 million people in the U.S.

If the projections prove true, millennials are set to become the largest generation in American history.

Baby boomers, previously named the largest generation in the U.S., saw their population peak in 1999 with 78.8 million people. Researchers with the Pew Research Center project the generation to dwindle to 16.6 million by mid-century.

Generation X, which falls between the boomer and millennial generations, remains the third largest and is expected to stay at that rank until 2028. In that year, researchers believe there will be 64.6 million Gen Xers, edging past the projected 63.7 million boomers.

The smaller number of Gen Xers is caused partially by the fact that the generation includes fewer birth years, 16, then the millennial generation, which has 17. Families were also having fewer children during the Gen X birth years between 1965 and 1980.

Still, the so called "middle child" of the generations is expected to grow further before reaching its peak with a population of 65.8 million in 2018.