It finally happened: This year, millennials surpassed baby boomers as the largest share of the U.S.'s voting-age population.

The U.S. now has 88 million millennials, people born 1981 to 2000. They are more than the sum of their student loans. This generation has tremendous political clout: Three of 10 voting-age Americans are millennials, and more members of the generation reach voting age each day.

A by-the-numbers look shows a generation poised to take over. Politicians ignore millennials at their peril.

Methodology

Generations, Defined

Generations are artificial creations. Who's to say that a 34-year-old is anything like a 15-year-old? Or that both of them should be deemed "millennials"? But delineating generational groups can help suggest how the U.S. has changed and will continue to evolve.

Baby boomers tend to be defined as Americans born 1946 to 1964. There's no such consensus for millennials. For our calculations, we chose 1981 as the birth year of the first millennials because that's the starting point the Pew Research Center uses. We selected 2000 as the last birth year, for a neat dividing line. (A 2015 report on "the demographic evolution of the American electorate" used the same definition.) That puts Generation X nestled between boomers and millennials: born 1965 to 1980. Some of the Pew data we used also includes Silents, who precede baby boomers, with birth years 1928 to 1945.

For the generations that follow millennials, we mimicked the millennial generation's 20-year span: Generation Z comprises those born 2001 to 2020, and Generation Post-Z extends 2021 to 2040. As for the names of those future generations, who knows? The millennial generation has also been called Generation Y; "Generation Z" seems to be getting some traction for the next group, but the jury is still out. "Generation Post-Z" is a lame, arbitrary name; we blame X for being so late in the alphabet.

Millennials Are Getting Older

"Millennial" is not synonymous with "young person." Millennials will always be members of the generation but won't always be young. Some of their traits may be more related to being young, or to being alive at a certain moment, than to being millennials; it's impossible to fully differentiate, but examining data over time can help.

Generations are moving targets; the range of ages they encompass changes. Data for a relevant age group can offer insight into a generation, but it's important to recognize its limitations. Data that is for, say, 18- to 29-year-olds in 2012 doesn’t include the oldest and youngest millennials (by our definition), while data for those 18 to 34 in 2013 both doesn't cover the youngest millennials and contains some members of Gen X. Moreover, even where Pew specifically tracks the millennial generation over time, it's only looking at those 18 and older — meaning that, up to this point, another year of the generation has been added to the mix with each subsequent year of polling. Pew has not yet settled on an endpoint for the millennial generation. (All Pew data used includes only adults 18 and older.)

Population Data

For our calculations on the generational shares of the overall and voting-age populations, we used Census estimates sliced by single year of age so as to follow generations over time. For the 2015-2060 data, we relied on data from Census's 2014 national population projections. All data is for the U.S. resident population, with two exceptions: Data prior to 1980 is for the resident population plus armed forces overseas, and data prior to 1950 excludes Alaska and Hawaii. (Because of the format of the Census projection data, starting in 2046 we counted all Americans 100 and over as boomers.)

We also used this data in calculating the ratios of young Americans to working-age Americans and older Americans to working-age Americans. We defined the working-age population as those 18 to 64, as opposed to, say, 15 to 64 — which seems a bit Dickensian.

In examining power at the polls, we looked at the U.S. voting-age population (everyone 18 and over) rather than just eligible voters. Certain subsets of the voting-age population — noncitizens, some felons — cannot vote. A calculation of eligible voters would be more complicated, especially given the possibility of future changes in immigration policy.

How Big Is Big?

Millennials are currently the U.S.’s biggest generation. But the baby boom generation made up a larger chunk of the overall population at its peak (in percentage terms) than the millennial generation did at its. Which is to say: In 1964 baby boomers were 37.8 percent of the U.S. population, while millennials were just 28.6 percent in 2000. Census's Sandra Colby has written: "Despite the larger size of the millennial generation relative to the baby boomers, their transition through the life course has not introduced (and, moving forward, is not as likely to introduce) the same level of shock to societal institutions as the baby boomers caused. This is because the baby boom was marked by large increases in birth cohorts relative to those that had come before. This is not the case for the millennials. The millennials are part of an ongoing trend toward larger birth cohorts that started in the previous generation and has continued into the next."

Party Politics

What’s the difference between identifying with and leaning toward a political party? Here’s how Pew — whose data we used for this — approaches it: To establish party identification, respondents are asked whether they consider themselves Republicans, Democrats or independents. Those who answer that they are independents, have no preference, identify with another party or don't know/refused to answer are then asked whether they lean more to the Republican or Democratic Party. The data underlying the assertion that millennials resist party identification derives from the first question, where a plurality of the generation identifies as independent. The data supporting the conclusion that millennials lean Democratic combines both questions; when adding those who identify as Democratic to those who lean Democratic, the generation lands on the left. Pew has also written that "even those Millennials who do identify as Republicans or lean toward the GOP are decidedly less conservative than older Republicans."

Growing Up

Are millennials delayed on the usual path to adulthood, or will maturity mean something different for this generation? The conventional notion of adulthood is not about age, but rather about classic markers of independence and stability: getting married, buying a house, having kids. Millennials may never buy houses or get married (or have kids?) at the same rates as their predecessors. (Measuring fertility is complicated.) Norms and desires change, and many millennials spent part of their early adult lives buffeted by the Great Recession. To what extent will any changes in attaining traditional "adult" status have derived from necessity versus preference?

Millennial Invincibles

Young people's participation is crucial to Obamacare's success. Millennials won't always be young people, but they have happened to fall into that designation at the law's outset. Looking at the uninsurance rate for 19- to 25-year-olds in recent years sheds light on how millennials have benefited from the health-care law. Indeed, the first beneficiaries of the law's provision allowing young people to be on their parents' plan until age 26 were millennials.

Why are young people important to Obamacare? Because they tend to pay more into insurance pools than they take out. And because they're a significant part of the work still to be done to decrease the overall uninsurance rate: According to U.S. Secretary of Health and Human Services Sylvia Burwell, going into the third open enrollment period, almost half of people who were uninsured and probably qualified for plans in the Health Insurance Marketplace were 18 to 34.

Children of Terror

It stands to reason that Sept. 11 should be a defining moment in the millennial generation's trajectory. And yet it's difficult to assess how the terrorist attack affected members of the generation: Most were too young to be polled in 2001, and now their thoughts are in retrospect. Likewise, examining attitudes about foreign policy over time can be challenging because current events can influence responses.

Political Engagement

Are young people "turned off to politics" and to running for office? A dearth of historical data on youth political ambition makes predictions about millennials' future in elected office somewhat difficult. And while the average age in both houses of Congress has risen, the U.S.'s population has gotten older, too.

The story of young-adult voting in recent years also isn't seamless. (Unlike in our electorate model, in which we examined the voting-age population, the Census data we used to show voting rates in recent presidential elections looks at turnout in the voting-age citizen population.) Young people voted at a relatively high rate in 2008. Was that a blip or a new pattern? After all, youth turnout was also high in 2004 compared with the prior two presidential elections. It's hard to tell where youth turnout landed in 2012: According to Tufts University's Center for Information and Research on Civic Learning and Engagement, there was a disparity between estimates using exit-poll voting data and using Census voting data (50 percent versus 45 percent turnout, respectively, for those ages 18 to 29). We'll have to see what happens in 2016.

Bloomberg Poll

A Bloomberg Politics poll provided insight in two areas: what Americans "see as the most important issue facing the country right now" and what young Americans view as "the main obstacle" to gun control. The data came from a landline and mobile-phone survey conducted Sept. 18-22 by Selzer & Co., including 402 adults identified as 18 to 35 and 819 other adults.

Acknowledgments

Special thanks to the Pew Research Center, the U.S. Census Bureau and the Chicago Council on Global Affairs.

Researcher: Alex Bruns, Bloomberg View, and Jennifer Prince
Editor: Philip Gray, Bloomberg View
Graphics: Jeremy Scott Diamond