Hillary Clinton's birthday puts her on path to beat...William Henry Harrison

Hillary Clinton’s is celebrating her 68th birthday Monday, meaning that, if elected, she will be the second-oldest president to take office — just a few months younger than Ronald Reagan was in January 1981.

The second-oldest president inaugurated thus far is William Henry Harrison, who took office at age 68.

Clinton is also competing in a potentially seasoned field.

The Republican frontrunner Donald Trump is already 69, and will turn 70 next June, his age upon a potential inauguration. Clinton’s chief Democratic rival, Vermont Sen. Bernie Sanders, would be 75 when sworn in should he actually get elected. The oldest of the candidates on the Republican side is former New York Gov. George Pataki, at 70. Two other GOP candidates, Sens. Ted Cruz of Texas and Marco Rubio of Florida, are both 44.

For the last week, Democratic supporters, elected office holders and others have been sending emails to supporters asking them to sign their name to a virtual birthday card and eventually to donate to the campaign.

“You can help make this a special birthday for Hillary. I know how much she'd appreciate seeing your name on her card,” said a campaign email purportedly from former President Bill Clinton Monday morning.

Other older presidents upon their inauguration were James Buchanan, 65; George H.W. Bush 64; Zachary Tyler, 64; Dwight Eisenhower, 62; Gerald Ford, John Adams and Andrew Jackson were all 61; and Harry Truman was 60.