What is the youngest that a US president has ever held an office at? I think that it's Andrew Johnson, who was elected an alderman at age 20?
Though I was kind of surprised by how young Franklin Pierce and Theodore Roosevelt were when they held pretty senior positions in their state legislature.
Yeah, I'm pretty certain it's Andrew Johnson when he was elected an alderman in his hometown of Greeneville, Tennessee in 1828. And, really, Johnson was 19 years old because his birthday is December 29th, so he was only 20 years old for the last two days of 1828. Johnson also became Mayor of Greeneville at the age of 21.
While it wasn't an elective office or a traditional political position, George Washington was appointed the surveyor of Culpeper County, Virginia when he was 17 years old. Washington received the position partly due to the political influence of Virginia landowner Lord Fairfax, who was a mentor. So, while it wasn't an elective office, Washington began to learn valuable political, financial, and leadership lessons as a teenage county surveyor. Franklin Pierce was remarkably young at every position he held: elected to the New Hampshire House of Representatives at 25 years old; elected Speaker of the New Hampshire House at 27 years old; elected to the U.S. House of Representatives at 29 years old; elected to the U.S. Senate at 33 years old; and he was 48 years old when he was inaugurated as President of the United States -- the youngest President in American history up to that time.
But when I took a deep dive after getting your question, I found that a surprising number of Presidents were elected or appointed to offices while they were in their 20s:
•John Tyler was elected to the Virginia House of Delegates when he was 21 years old. •Andrew Jackson was made the public prosecutor for the Western District of North Carolina in 1788 when he was 21 years old. •James Madison was elected to the Orange County (Virginia) Committee of Public Safety when he was 23 years old. •James Monroe was elected to the Virginia House of Delegates when he was 24 years old. •James Buchanan was elected to the Pennsylvania House of Representatives at 24 years old. •Benjamin Harrison was elected City Attorney of Indianapolis at 24 years old. •Theodore Roosevelt elected to the New York State Assembly at 24 years old. •William Howard Taft was assistant prosecutor of Hamilton County, Ohio at 24 years old. •James K. Polk was clerk of the Tennessee State Senate at 24 years old and elected to the Tennessee House of Representatives at 28. •Abraham Lincoln was Postmaster of New Salem, Illinois at 24 years old and elected to the Illinois State Legislature at 25. •William Henry Harrison was appointed Secretary of the Northwest Territory at 25 years old. •Thomas Jefferson was a member of the Virginia House of Burgesses at 26 years old. •Martin Van Buren was appointed surrogate of Columbia County, New York at age 26. •William McKinley was elected prosecutor of Stark County, Ohio at 26 years old. •Grover Cleveland was assistant district attorney for Erie County, New York at 26 years old. •John Quincy Adams was appointed U.S. Minister to the Netherlands by George Washington when JQA was 27 years old. •Calvin Coolidge was elected to the Northampton, Massachusetts City Council at 27 years old. •Lyndon B. Johnson was Director of the National Youth Administration in Texas at 27 years old. (LBJ also was the extraordinarily influential secretary to Texas Congressman Richard M. Kleberg and practically ran that Congressional district when he was 23 years old.) •James Garfield was elected to the Ohio State Senate at 28 years old. •Millard Fillmore was elected to the New York State Assembly at 29. •Franklin D. Roosevelt was elected to the New York State Senate at 29 years old. •Joe Biden was 28 years old when he was elected to the New Castle County (Delaware) Council. He also was 29 years old when he was first elected to the U.S. Senate, so he wouldn't have been old enough to actually take his seat if he had been sworn in immediately.