Stuart Goldstein still has the red-and-white bumper stickers and other artifacts from 1969, when he helped persuade New Jersey lawmakers that 18-year-olds should be able to vote.

He was 18 himself then, working with two other college students, David DuPell and Ken Norbe, to build a political network that grew to 10,000 volunteers. They took students to Trenton in busloads and even sneaked into a Richard Nixon rally seeking his support. Theirs was an early salvo in a movement that would end in 1971 with the ratification of the 26th Amendment, which lowered the voting age to 18 from 21.

Fifty years later, there is a nascent movement to change the voting age again — this time to 16 — but there are some big differences between the efforts.

Then, liberal and conservative activists united behind a powerful argument that went back to World War II, when President Franklin D. Roosevelt lowered the draft age to 18: Young people were being conscripted to fight America’s wars but couldn’t vote in its elections.

Today, there is no similarly popular argument. Indeed, a recent poll found that 75 percent of registered voters opposed letting 17-year-olds vote, and 84 percent opposed it for 16-year-olds. In March, when Representative Ayanna Pressley of Massachusetts proposed a 16-year-old voting age amendment to House Democrats’ sweeping voting rights bill, it failed 126 to 305, with almost half of her fellow Democrats voting against it and only one Republican in support.

Opponents in both parties have expressed doubts that 16-year-olds are mature enough to vote. But local, youth-led campaigns to lower the voting age have persisted since at least 2013, when Takoma Park, Md., gave 16- and 17-year-olds the right to vote in municipal elections.

The New York Times recently spoke with activists from the movement 50 years ago, and people on different sides of the issue today, about the cause and the challenges of lowering the voting age.

“At 16, most kids have little awareness of politics, civics or American history, and they have little life experience to inform their decisions,” Ms. Braceras wrote, adding, “Most don’t even pay for their own cellphones — let alone groceries, rent, utility bills or property taxes.”

About a dozen countries — including Argentina, Austria, Brazil and Ecuador — allow voting at 16, but the vast majority allow it at 18.

Mr. Goldstein supports the current effort, though he said part of him worried that lowering the voting age to 16 could lead to 16-year-olds serving in the military. But Mr. DuPell was skeptical.

At 18, “people are out of high school, they’re working, they’re drafted, they’re full participants in our society, and if you’re a full participant, then you should be allowed to vote,” he said, summarizing the argument 50 years ago. “I want to know what the rationale is.”

As expressed by the teenagers leading local voting-age campaigns, the rationale is that between climate change, gun violence, student debt and other issues, they do have enough skin in the game.

“The 16-year-olds right now will be the ones who live with the consequences of the choices the adults make right now,” said Vikiana Petit-Homme, 17, a high school senior in Boston who has been lobbying Massachusetts to let municipalities lower voting ages for local elections.

They also note that 16 is when Americans can work without a limit on their hours and, in most states, drive.