This was an age that could not afford to “put down” the young. Over the second half of the 1800s, American politics darkened. The succession of Civil War, Reconstruction and the Gilded Age was marked by bitter partisanship, endemic corruption, appalling violence and a general sense that democracy was failing. Adults seemed to be stuck, recycling the same partisan issues over and over.

And they insisted that the young follow their lead. As an established Minnesota politico told one rising 25-year-old: Don’t think, just follow the party’s platform, just “swallow it down.”

But in the final years of the 19th century, a sudden burst of young people demanded new issues — their issues. Tired of, as one Coloradan put it, “rotten old hulks who monopolize the offices and dwell upon the past,” a generation of young men and women denounced their leaders and with them, partisanship. They demanded political reform, labor reform and social reform, and declared that they would withhold their votes from any party that didn’t respond. “The ratio of party feeling and self-interest is rapidly changing,” declared one sharp-tongued New Yorker in 1898, adding that “the younger generation hates both parties equally.”

Politicians saw the change and chased after those young voters. Soon, The Washington Post was begging: “Don’t sneer at them as ‘boys,’ when they drop into your ward meetings, don’t make them do all the work of the campaign.”

In the new century, young people’s “self interest” helped kill extreme polarization by forcing both parties to pursue the same set of demands. Youthful independent voters emerged as a decisive third force, with just enough influence to swing close elections. Politicians scrambled after them, beginning the Progressive Era, passing laws protecting workers, cleaning up cities and championing the young.

Women played a key role in this shift. Because they could not vote, they were less corrupted by partisanship. Women in their 20s worked to refocus American public life toward social concerns. They built schools — nearly one a day between 1890 and 1920 — and fought child labor. The aggressive crusader Florence Kelley wrote that protecting young people is “the noblest duty of the Republic,” an act of “self preservation” that enables the next generation to champion itself.

This is the key to understanding youth politics. Young people cannot be truly selfish, because they cannot be permanently young. Youth is temporary, its gains are passed on. In the late 19th century, young people built institutions that helped protect 20th-century young people. The high school seniors marching over the weekend will hopefully make their schools safer well after they have graduated. If anyone should be choosing the issues in politics, it should be the young, for they “have a longer future to provide for.”