The United States grew older, faster, in the last decade.

The share of residents 65 or older grew by more than a third from 2010 to 2020 and at the fastest rate of any decade in 130 years, while the share of children declined, according to new figures from the most recent census.

The declining percentage of children under age 5 was particularly noteworthy in the figures from the 2020 headcount released Thursday. Combined, the trends mean the median age in the U.S. jumped from 37.2 to 38.8 over the decade.

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America's two largest age groups propelled the changes: more baby boomers turning 65 or older and millennials who became adults or pushed further into their 20s and early 30s. Also, fewer children were born between 2010 and 2020, according to numbers from the once-a-decade head count of every U.S. resident. The decline stems from women delaying having babies until later in life, in many cases to focus on education and careers, according to experts, who noted that birth rates never recovered following the Great Recession of 2007-2009.

"In the short run, the crisis of work-family balance, the lack of affordable child care, stresses associated with health care, housing, and employment stability, all put a damper on birth rates by increasing uncertainty and making it harder to decide to have and raise children," said Philip Cohen, a sociologist at the University of Maryland.

There are important social and economic consequences to an aging population, including the ability of working-age adults to support older people through Social Security and Medicare contributions. The Census Bureau calculates a dependency ratio, defined as the number of children plus the number of seniors per 100 working-age people. While the dependency ratio decreased for children from 2010 to 2020, it increased for seniors by 6.8 people.

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At the top end of the age spectrum, the number of people over 100 increased by half, from more than 53,000 people to more than 80,000. The share of men living into old age also jumped. Buddy Lebman, a 98-year-old in the St. Louis area, said the key to longevity is good genes and staying active. He plays bridge twice a week, leads a discussion on current events at his retirement community, and is still involved with his synagogue and a school he helped found. Up until five years ago, he went on regular bicycle rides.

"I just recently had my pacemaker checked out, and the doctor told me it's good for 4 1/2 more years," Lebman said. "So I have to live at least that amount."

People reaching age 100 benefited from a century of vaccines and antibiotic developments, improvements in surgery and better treatment of diseases, said Thomas Perls, a professor of medicine at Boston University.

"Many more people who have the genetic makeup and environmental exposures that increase one's chances of getting to 100, but who would have otherwise died of what are now readily reversible problems, are able to fulfill their survival destiny," Perls said.

The Census Bureau released two earlier data sets from the 2020 census in 2021: state population figures used to decide how many congressional seats each state gets and redistricting numbers used to draw political districts. Thursday's data release was delayed by almost two years because of pandemic-related difficulties gathering the information and efforts by the Census Bureau to implement a new, controversial privacy protection method that uses algorithms to add intentional errors to obscure the identity of any given respondent.

This was the first census since the U.S. Supreme Court legalized same-sex marriage in 2015. The tally showed that more than half of U.S. households contained coupled partners or spouses who lived together, and same-sex households made up 1.7% of those households. Since the census didn't ask about sexual orientation, it didn't capture LGBTQ+ people who are single or don't live with a partner or spouse.

The median age varied widely by race and ethnicity. Non-Hispanic whites were the oldest cohort, with a median age of 44.5. Hispanics were the youngest, with a median age of 30; and a quarter of all children in the U.S. were Hispanic. Black Americans who weren't Hispanic had a median age of 35.5. The number was for 37.2 for Asians.

Utah, home to the largest Mormon population in the U.S., was the youngest state, with a median age of 31.3, a function of having one of the nation's highest birthrates. The District of Columbia's median age of 33.9 was a close second due to the large number of young, working-age adults commonly found in urban areas. North Dakota was the only state where the median age declined, from 37 to 35.8, as an influx of young workers arrived to work in a booming energy sector.

Maine was the oldest state in the U.S., with a median age of 45.1, as more baby boomers aged out of the workforce. Puerto Rico had a median age in the same range, at 45.2, as an exodus of working-age adults left the island after a series of hurricanes and government mismanagement. Older adults in four states — Florida, Maine, Vermont and West Virginia — made up more than a fifth of those states' populations.

Sumter County, Florida, home of the booming retirement community The Villages, had the highest median age among U.S. counties, at 68.5; while Utah County, home to Provo, Utah, and Brigham Young University, had the lowest at 25.9.

As one of the youngest baby boomers, Chris Stanley, 59, already lives in The Villages. She said her mission in later life is to let younger generations know they can change things despite perhaps not having the same economic opportunities she did.

"I want to impart the urgency that I feel," she said. "They can make it better."

While people 65 and older made up 16.8% of the 331 million residents in the U.S. in 2020, the share was still significantly lower than it was in countries like Japan, Italy and Greece, where the age cohort makes up between more than a fifth and more than a quarter of the population. However, their share of the U.S. population will continue to grow as baby boomers age.

"In the long run, immigration is the only way the United States is going to avoid population decline," Cohen said.

WASHINGTON, D.C. — The Census Bureau has issued its most detailed portrait yet of how the U.S. has changed over the past decade, releasing a trove of demographic data that will be used to redraw political maps across an increasingly diverse country.

The bureau held a press conference explaining the data on Thursday:

The census figures have been eagerly awaited by states, and they are sure to set off an intense partisan battle over representation at a time of deep national division and fights over voting rights. The numbers could help determine control of the U.S. House in the 2022 elections and provide an electoral edge for the next decade. The data will also shape how $1.5 trillion in federal spending is distributed each year.

The figures show continued migration to the South and Southwest and population losses in the Mississippi Delta and Appalachia. The numbers also indicate that the white population is aging and has fallen to its smallest share of the total population on record, though there are some exceptions. The share of the white population actually grew in coastal communities in the Carolinas and Virginia, as well as in counties stretching through the midsections of Georgia and Alabama. The population under age 18 is increasingly diverse.

The data comes from compiling forms filled out last year by tens of millions of Americans, with the help of census takers and government statisticians to fill in the blanks when forms were not turned in or questions were left unanswered. The numbers reflect countless decisions made over the past 10 years by individuals to have children, move to another part of the country, or to come to the U.S. from elsewhere.

The release offers states the first chance to redraw their political districts in a process that is expected to be particularly brutish since control over Congress and statehouses is at stake. It also provides the first opportunity to see, on a limited basis, how well the Census Bureau fulfilled its goal of counting every U.S. resident during what many consider the most difficult once-a-decade census in recent memory.

Even before it began, the headcount was challenged by attempted political interference from the Trump administration’s failed efforts to add a citizenship question to the census form, a move that critics feared would have a chilling effect on immigrant or Hispanic participation. The effort was stopped by the Supreme Court.

The information was originally supposed to be released by the end of March, but that deadline was pushed back because of delays caused by the coronavirus pandemic.

The start of the 2020 census for most U.S. residents coincided with the spread of the coronavirus last year, forcing the Census Bureau to delay operations and extend the count’s schedule. Because census data is tied to where people were on April 1, 2020, the numbers will not reflect the loss of nearly 620,000 people in the U.S. who died from COVID-19.

On top of the pandemic, census takers in the West contended with wildfires, and those in Louisiana faced repeated hurricanes. Then, there were court battles over the Trump administration’s effort to end the count early that repeatedly changed the plan for concluding field operations.

Back in April, the Census Bureau released state population totals from the 2020 census showing how many congressional seats each state gets.

“Certainly, the pandemic played a big role, but we can’t forget the political interference we saw,” said Terry Ao Minnis, an official with Asian Americans Advancing Justice, an advocacy group. “I think we know that all has played a role in whether people participated or not, whether it was from fear created about participating or sheer confusion about, ‘Who is at my door? ... Should I not open my door because of COVID? Should I not open my door because of the government?’”

Communities of color have been undercounted in past censuses. The Census Bureau likely will not know how good a job it did until next year, when it releases a survey showing undercounts and overcounts. But Thursday’s release allows researchers to do an initial quality check, and it could lead to lawsuits alleging that the numbers are faulty. The Census Bureau has a program that allows elected officials to challenge the data, but it does not apply to apportionment or redistricting.

“This is our first opportunity to see if there’s any indication of an unprecedented undercount,” said Thomas Saenz, president of the Mexican American Legal Defense and Educational Fund (MALDEF). “There always is an undercount. This census will be no different, but our concern is to make sure this isn’t hugely out of proportion to undercounts we have seen in prior censuses.”

For the first time, the numbers will not be entirely accurate at the smallest geographic levels due to a new privacy method used by the Census Bureau. The method inserts controlled errors into the data at small geographic levels, such as neighborhood blocks, in order to protect people’s identities in an era of Big Data.

Acting Census Bureau Director Ron Jarmin has warned that the process may produce weird results, such as blocks showing children living with no adults or housing units not matching the number of people living there.