The Founding Fathers of the United States were children of the 18th century. Even Benjamin Franklin, one of the oldest (born in 1706), was born in the same century in which the U.S. government was founded.
George Washington, born in 1732, never lived to see the 19th century, passing away in December 1799, a few weeks before it began.
Since independence from Britain was not declared until 1776, it's not surprising that no U.S. political leader of the time had been born in the previous century. The Constitutional requirement that a president be at least 35 years of age also creates some necessary distance in time between when a future president is born and the decade or century he may hold power.
The 8th president, Martin Van Buren, inaugurated in 1837, was the first U.S. president to be born after 1776. (He was born in 1782.) His successor, William Henry Harrison, who died after a month in office, was the last U.S. president born before 1776. (He was born in 1773.)
The first U.S. president to be born in the 19th century was Millard Fillmore, who was born in January 1800. Fillmore had been vice-president, and became the 13th president in July 1850 after the death of Zachary Taylor. So, it was not until the 19th century was half over that a person born in that century became president.
When Fillmore died in 1874, he and Andrew Johnson (who died the following year) had been the only living ex-presidents. When Johnson's successor Ulysses S. Grant left office in 1877, Grant found himself to be the only living ex-president. (Nearly 100 years later, when Richard Nixon left office in 1974, there were also no living former presidents other than himself.)
Fillmore's successor, Franklin Pierce, was also born in the 19th century (1804), but Pierce's successor was an older man, James Buchanan, who was born in 1791. Leaving the office in 1861, Buchanan was the last U.S. president to be born in the 18th century.
The U.S. Civil War began in 1861, but it was not until 60 years later, when Warren G. Harding was inaugurated in 1921, that the country had a president born after the Civil War. (Harding was born in 1865, several months after the war ended.) All of Harding's predecessors had been born before the war started, and all the presidents that followed were, like him, born after it had ended. (None were born during the war.)
Just as James Buchanan became the last U.S. president born in the 18th century, when he was inaugurated in 1857, exactly 100 years later Dwight D. Eisenhower (born in 1890) became the last U.S. president born in the 19th century when he was inaugurated (for his second term) in 1957.
John F. Kennedy, inaugurated in 1961, was the first U.S. president to have been born in the 20th century -- as all subsequent U.S. presidents have been. (He was born in 1917.)
Kennedy was the first president to have been born in the same century in which he became president since William McKinley (who was born in 1843 and was president from 1897 to 1901). Like McKinley, Kennedy was assassinated while in office.
Looking at the above history, we see that it took 50 years in the 19th century for someone born in that century to become president (Millard Fillmore in 1850) and it took 61 years in the 20th century for someone born in that century to become president (John F. Kennedy in 1861).
If the 21st century conforms to a similar pattern as the previous two centuries, then we can expect that it will not be until 2050 or 2060 that someone born in the 21st century will become president. Which means that half of the current century will be led by a president whose formative years took place in the previous century.
Living through an era, as opposed to reading about it later in a history book, can profoundly shape a person's outlook and identity. The generation that lived through World War Two, for example, will recall the sacrifices and battles of that time, which were experienced by their fellow countrymen and understood implicitly like a common language that they share. Over time, as new generations are born, the names of places where battles were fought and lives lost fade from the nation's collective memory, as fewer and fewer among the population were alive to experience them.
World War Two (1939-1945) was perhaps the most important event of the 20th century. America's experience in liberating Europe from the Nazis undoubtedly influenced military decisions by American presidents in subsequent decades, usually with less fruitful results.
Until 1993, when Bill Clinton was inaugurated, every U.S. president had been born prior to 1925. Every president since Herbert Hoover had been alive during the Great Depression and World War Two. Clinton, however, was born in 1946 and his outlook growing up was shaped by the civil rights and anti-war movements of the 1960s.
Bill Clinton was the last U.S. president to govern during the 20th century. His successor, George W. Bush, inaugurated in 2001, was born the same year as Clinton was, 1946. Donald Trump was also born in 1946. The three presidents were born within three months of each other: Trump on June 14, Bush on July 6, and Clinton on August 19. (Two recent nominees who failed to win the presidency, Mitt Romney and Hillary Clinton, were born the following year, 1947.)
It's often noted how presidents utilize the technology of their times, such as FDR's mastery of radio with his fireside chats, or JFK's use of television for his press conferences and debates. The medium of radio and TV were not exactly brand-new inventions, but young and growing industries, and their use by these leaders for political purposes impacted how they would be used by those that followed. Indeed, a media-savvy presence would become essential for political success, culminating in the election of a former movie and TV star as president in 1980 with Ronald Reagan.
Social media on the internet is a uniquely 21st century innovation that has also taken on a dominant role in political discourse, influencing (and even originating) news coverage. Twitter was founded in 2006, and citizen/celebrity Donald Trump created an account on the site in 2009. Given the media's use of Twitter in the formation of their news stories, Trump was able to use the site as a way of sending unfiltered messages to the public while simultaneously remaining in the media spotlight. This was very much a 21st century phenomenon, and it could be argued that Trump was the first true "21st century president" (no matter the year of his birth) by exploiting this new method of communication.
However, we are only 21 years into the 21st century -- at a point comparable when Warren G. Harding became the first post-Civil War president in 1921 -- and there's no telling how the rest of the 21st century will look. It's just possible that it may look more like Trump's predecessor, Barack Obama.
Obama, America's first African-American president, was unlike any president before or since, due to his racial identity. He was also born more recently (1961) than the others, and will become only 60 years old later this year. For Obama, the events of the Great Depression and World War Two are even more remote than they were for his fellow baby boomers Clinton, GWB and Trump, all born in 1946. "Baby boomers" is the term for those people born between 1946 and 1964, a group which also includes the current Vice-President Kamala Harris (if not President Joe Biden himself, who was born in 1942).
Kamala Harris was born in 1964. It seems reasonable to assume that at some point she will run for the presidency, and if so would likely be a strong frontrunner given her current position as vice-president. (Harris ran in 2019, but dropped out before any votes were cast.) As president, she would not only be the second African-American president, but the first woman to be president. As the demographics of the country become more diverse, a president that reflects such diversity would be more representative of the century in which they live, and more important a factor than whether they use Twitter or not.
Another prominent female politician these days is Alexandria Ocasio-Cortez, born in 1989. Like Trump, she has used social media to enhance her visibility and voice. AOC (as she is known) also has a Puerto Rican heritage, an ethnic background than reflects the country's growing diversity. Currently only 31 years old, AOC would turn 35 on October 13, 2024, which means that if she ran for president she would meet the Constitutional requirement of being 35 years or older when taking office. (The 2024 election will take place on November 5, 2024.)
If such a thing happened, she would be the youngest person to ever hold the office of U.S. president. (At age 42, Teddy Roosevelt was the youngest person to become president, assuming office upon the death of President McKinley.) Nonetheless, a President AOC would still be a "20th century" person leading the country in the 21st century. Nearly one-third of her life would have been lived in the previous century.
Whoever becomes the Millard Fillmore or John F. Kennedy of the 21st century (i.e., the first president to be born in the same century in which they became president), it would have to be someone born after the year 1999. This unknown person is currently 21 years old or younger, and (as noted earlier) will likely not become president until 2050 or later. Constitutionally, they could not even run for president until 2035 at the earliest. Which means we probably have another 30 to 40 years of presidents who were born in the 20th century until that happens.
However, I suspect it will happen sooner than 2050. In the past, experience in either governing or military leadership were considered important factors in the election of a president. Recent history has shown these qualities to be less important -- and perhaps even disadvantages -- than in the past. For example, both Bill Clinton and Barack Obama won their respective elections against opponents with admirable military backgrounds (Bush, Dole, McCain). Donald Trump was the first president to be elected who was neither a politician nor military leader.
The most important factor in winning elections may simply be an ability to create favorable impressions of one's self, which is the guiding principle of social media sites like Instagram (founded in 2010). The power to control the narrative, to manipulate the populace's understanding of reality in the promotion of one's own ends, has long been important in politics, as mentioned above regarding FDR on radio and JFK on TV.
Using the "bully pulpit" (a phrase coined by Teddy Roosevelt) to achieve one's goals is nothing new. But now each person has their own online pulpit in which to spread their gospel, in the hope of winning converts to their cause, regardless of whether the message is literally true or not. A polarized electorate chooses sides in the manner of watching a football game, based on traditional loyalties, their interest maintained by commercial hype and the dramatic action on the playing field. The bolder the play, the bigger the cheer from the crowd, and the greater the media exposure.
Politics is now just another sporting event, just another movie premiere, just another viral meme, for a shallow and bored public. It's possible that a future president, the product of such a culture, could debase the office in ways unimaginable, even to those who have lived through Watergate, through the Clinton sex scandals, and through the reckless rhetoric of Trump. I guess we'll just have to wait and see what the future holds, and hope for the best.