When Joe Biden first came to the Senate in 1973, he was the youngest member of that body. Among the oldest was Vermont’s Sen. George Aiken, who had joined the Senate in 1941, the year before Biden was born. Aiken himself was born in the 19th century — 1892, less than 30 years after the end of the Civil War. But there he was, serving alongside the 31-year-old Biden.

That Biden is now president, nearly half a century after coming to the Senate, serves as a remarkable bridge between centuries. But it also highlights how young the United States is — that the current president could have been in political proximity to legislators who were living at the turn of the last century.

In fact, as we’ve noted before, the United States is only four presidents’ lifetimes old.

Here’s how that works. We can use presidents’ birth and (when applicable) death dates to determine which presidents were alive during which other presidents’ lifetimes. No president overlapped with more of his peers than did Rutherford B. Hayes, who was alive for all or part of 31 other presidents’ lives, from John Adams to Dwight D. Eisenhower. (Adams died four years after Hayes was born; Hayes died three years after Eisenhower was born.) No president overlapped with fewer than George Washington.

Using the signing of the Declaration of Independence — July 4, 1776 — as the birth date of the country, we can see how it takes only four presidents to stretch back that far. Biden was born during the lifetime of Herbert Hoover. Hoover was born during the lifetime of Andrew Johnson. Johnson was born during the lifetime of John Adams, who signed the Declaration.

It takes four presidents to stretch back to Washington’s lifetime as well (predictably, given that he, too, was alive in 1776). You can pick any number of combinations; below, we show a Biden to Hoover to Hayes to John Tyler to Washington chain.

Here’s how that looks as an actual timeline.

You’ll notice that the same patterns hold for most recent presidents. Every president since Hoover left office has been alive during Hoover’s lifetime; he died in 1964.

In fact, he died on the day that Kamala D. Harris was born. But they didn’t overlap: He died in New York at 11:35 a.m., while she was born around 9:30 p.m. in Oakland, Calif. — technically on Oct. 21, 1964, Eastern time.

Should Harris succeed Biden as president, then, the connection back to the earliest days of the Republic will have to change. But only slightly. Harris was alive at the same time as Harry Truman, who was alive at the same time as Ulysses S. Grant, who was alive at the same time as John Adams and John Tyler.

The United States will be four presidents old for some time to come.