SCOTTS VALLEY >> In just under two months, millions of Americans will be casting a ballot in the U.S. presidential election. That includes many new voters who have turned 18 since the last presidential election.

Once upon a time — prior to the 1972 election, in fact — this was not the case. Americans had to be at least 21 to vote in elections, but the ratification of the 26th Amendment in 1971 created a whole new bloc of voters. Author Jennifer Frost goes over the history of the amendment and movements leading up to it in her most recent book “Let Us Vote,” which she will be discussing at the Scotts Valley Branch Library Thursday evening.

An associate professor of history at the University of Auckland in New Zealand, Frost has written several books about American history and culture including “‘An Interracial Movement of the Poor’: Community Organizing and the New Left in the 1960s,” “Hedda Hopper’s Hollywood: Celebrity Gossip and American Conservativism” and “Producer of Controversy: Stanley Kramer, Hollywood Liberalism.” “Let Us Vote!” goes into the history of why Americans are able to vote as young as 18.

While the framers of the Constitution did not designate a standard national voting age, many states established 21 as the youngest age Americans could vote. There had been movements dating back to the ’40s to lower the voting age to 18, which was exacerbated by the Vietnam War where many pointed out the discrepancy between being able to serve in the military as young as 18 but not having any say in voting for politicians who had the ability to declare war. The debate over lowering the voting age became widely talked about in the political sphere and even made its way into popular music, including Boyce and Hart’s “L.U.V. (Let Us Vote),” which was performed on “American Bandstand” in 1968 and Barry Maguire’s No. 1 hit “Eve Of Destruction,” where he laments — among other things — that, “You’re old enough to kill but not for votin’.”

The efforts were finally set into motion in 1970 when Massachusetts Sen. Ted Kennedy proposed amending the Voting Rights Act of 1965 to lower the voting age to 18, which then-President Richard Nixon signed an extension for. Despite several challenges, even going before the Supreme Court in Oregon v. Mitchell, the amendment to the lower the voting age to 18 passed in both houses of Congress and was ratified to the Constitution as the 26th Amendment.

Frost will dive deeper into the history of this movement, particularly how California played a role.

The event is 6:30 p.m. Thursday in the Fireside Community Room at the Scotts Valley Branch Library, 251 Kings Village Road. Registration is required and can be done at Santacruzpl.libcal.com/event/12892186.