When it comes to electoral outcomes, however, young people are being outflanked: “Rather than increasing their influence in 2020, what’s happened is, their parents and grandparents have increased their influence,” he said.

Those Democratic grandparents, especially, tend to be more moderate, more swayed by traditional government experience and more keenly focused on the tactics they believe are needed to defeat President Trump, strategists and pollsters said.

Mr. Biden, who once faced significant competition for older Americans, emerged in recent weeks as the dominant front-runner among those highly committed Democratic voters who have now helped bring him to the cusp of his party’s presidential nomination.

Older voters have punched above their political weight for years, with turnout among those 65 and older often double, or more, that of the youngest voters. As Americans age and become more rooted in their communities, political participation tends to rise with their stake in society.

Even in the midterm elections in 2018, hailed as a high-water mark for youth voting because the share of 18- to 24-year-olds nearly doubled from the previous midterm election, the gap with older voters remained about the same. About 66 percent of eligible older people turned out, compared with about 36 percent of 18- to 24-year-olds, said William Frey, a demographer at the Brookings Institution.

“There’s no magic age for becoming a regular voter,” said Carroll Doherty, the director of political research at the Pew Research Center. “But when people move into their 40s, that’s when you see voter turnout grow.”

Certainly, Mr. Sanders, the overwhelming favorite with younger voters, is continuing to campaign. And while the Vermont senator has acknowledged that younger voters did not appear to turn out at the rate he had hoped for, polls and exit surveys show that Mr. Biden faces major challenges with that constituency, a liberal slice of the electorate that, his advisers acknowledge, he will need to energize if he is the nominee.