Fifty years ago, when young Americans aged 18 to 20 got to vote in their first presidential election, Democrats were confident that it would be a bonanza for their candidate, George McGovern. But President Richard M. Nixon had other ideas. After the ratification of the 26th Amendment in 1971, which lowered the voting age from 21 to 18, Nixon invited 500 newly eligible voters to the White House for a July 5, 1971, ceremony, where the amendment was officially certified. “A country throughout history, we find, goes through ebbs and flows of idealism,” Nixon told them. “Time after time the...