From a New York Times article on how big voting was among 20-somethings in the 19th Century:
But something went wrong after 1900. Turnout crumbled, from roughly 80 percent in 1896 to 48.9 percent by 1924, as new voters stopped joining. With a stable school system and a welcoming teenage culture, youths needed politics less. At the same time, elite campaigners abandoned working-class young people, preferring hotel banquet halls to public barbecues. There were, to be fair, fewer stabbings on Election Day, but also fewer bonfires on election night.As my late friend, historian Jim Chapin pointed out, there wasn’t all that much besides politics to do for entertainment in 19th Century America. Movies and spectator sports came along after 1900 to give people something to talk about besides politics. Being a pop music star, like Jim’s brother Harry, wasn’t really a job in the 19th Century.
Youth politics never recovered. Even after the 26th Amendment lowered the voting age to 18 in 1971, turnout tumbled further.
Also Ellis Island immigrants didn’t identify with voting the way 19th Century Americans had.
And the last year of >75% turnout, 1896, was the year of the 36-year-old orator William Jennings Bryan, a prodigy of democracy. In losing in 1896, the prairie populist earned almost a million more votes than any winner had ever received in any previous election. East Coast snobs like H.L. Mencken have succeeded in blackening Bryan’s image, but his 1896 campaign was one of the great adventures of democracy.