While it is impossible to predict the future, experts say that population changes tend to follow patterns. But the personality of a generation can influence society, too.
The Baby Boomers make up the largest generation by far compared to anything that came before or after it.
As the Boomers push through the age-65 barrier, they promise to put economic strain on society.
Watch out next for the Millennial Generation, which is in its young adulthood now and is already having large impacts on society.
As the Baby Boomers begin to turn 65 this year, their generation is getting a renewed flood of attention -- not for their rebellious youths but for their impacts on the economy as they transition into retirement over the next 20 years.
But while the Boomers continue to steal headlines, younger cohorts are aging, too. And that raises questions about what to expect from the generations that follow. Where are the next demographic swells and valleys? And how will they reshape the country as they, too, march through life?
While it is impossible to predict what the future holds, experts say that population dips and swells tend to follow cyclical patterns, with predictable consequences as each cohort enters and exits each life stage. But the personality of a generation can influence society, too. And the Millennial Generation, which is coming of age right now, is already making its mark.
As for scope, the Boomer Generation remains in a class of its own. And its impending retirement promises to tip the balance, in an unprecedented way, toward an older nation.
"There's nothing remotely like the Baby Boom in terms of magnitude," said Ron Lee, a demographer and economist at the University of California, Berkeley. "There hasn't been anything else like that in U.S. history."
"Sudden changes in the number of births cause problems all down the line," Lee added. "Baby Boomers kept the population young. They filled the labor force with taxpayers and made life easier. Now the end of that life has come. The onset of aging is happening very rapidly."
Around 1800, at the beginning of modern American history, the average woman in the United States gave birth seven times, according to U.S. Census Bureau statistics. Fertility rates, which refers to the number of live births per woman, then declined throughout the 19th century and continued to go down during the roaring 1920s before finally bottoming out during the Great Depression and the beginning of World War II, when young men became scarce. By 1940, women were having an average of just 2.2 babies each.
As the war ended and soldiers returned home, fertility rates picked up again. 1946 marks the official beginning of the Baby Boom Generation, which lasted until 1964. Yearly ups and downs punctuated the period. But fertility rates generally rose to a peak of about 3.75 births per woman in the late 1950s. Over the same period, the number of live births in the U.S. rose from about 2.6 million in 1940 to 4.3 million in 1960.
But population numbers in the U.S. have never followed a straight line. A sharp drop in baby making followed the Boom and lasted through the '70s. By 1980, total numbers of births were down to 3.6 million. And in the early '80s, women were having an average of just 1.8 babies each.
Since the '80s, numbers have gone up and down several times. But trends have generally plateaued at just over two births per woman and 4.1 million births per year since the late 1990s. Population graphs show blips, such as a small drop in births during the recent Great Recession. But nothing comes close to being as extreme as the Baby Boom.
Numbers and graphs don't tell the whole story, though. As the Boomers reach their natural end, the next group to watch out for will be the Millennials, which is the generation of young adults born after 1981. Now ages 18 to 30, the Millennials make up the largest generation since the Boomers, said Paul Taylor, co-author of a Pew Research Center report called, "Millennials: A Portrait of Generation Next," published in February.
And like the Boomers, who once made a name for themselves challenging authority and rallying for women's rights to the backdrop of sex, drugs and rock 'n roll, today's young adults are calling attention to themselves, too.
Members of the Millennial Generation are the first to be natives of the digital world, Taylor said, and they are always connected. They are confident. They are well educated and not very religious. They're also more ethnically and racially diverse than previous generations. They get along with their parents. But they were twice as likely to vote Democratic in the 2008 election as older groups.
Today's young adults are entering the job market in tough economic times. Yet far fewer of them are joining the military compared to Boomers at the same age, the Pew Research study found. And despite the challenges, they remain upbeat about the future.
What the long-term future will bring is anyone's guess. The immediate future, on the other hand, guarantees at least some tension between generations. Thanks to the Boomers, Taylor said, the percentage of the American population that is older than 65 will surge from today's level of about 12 percent to close to 20 percent in two decades.
In turn, there will be a sharp rise in a measurement called the dependency ratio, which is the proportion of children younger than 18 and older folks above age 65 in comparison to people of working age. These kinds of shifting demographics promise put strain on Medicare, Social Security and other services.
"The dependency ratio was much higher than it is now in the '50s and '60s when all of the Boomers were under age 18," Taylor said. "It's come down dramatically in the last three to four decades because the Boomers were working. Now, as the Boomers crash through age 65, the dependency ratio is going way up again."