– As it celebrates another birthday, the United States is forging the outlines of a new ­century.

It’s moving with remarkable speed to cast aside some of the traditions and mores that dominated American life for centuries. Marriage is being redefined. The Confederate flag is coming down, 150 years after the end of the Civil War and a half-century after it was raised in defiance of civil rights. Whites are fast becoming a ­minority.

It is driven by a new generation, magnified by demographic change and accelerated by social media.

The biggest influence is the millennial generation, born between 1982 and 2000, a massive group that represents about one-fourth of the nation’s population. The millennials are the products of attitudes honed in a pervasive popular culture, where people of color and different sexual orientations fit smoothly into mainstream society.

“There’s a nonjudgmentalism that’s become clear,” said Karlyn Bowman, a senior fellow at Washington’s American Enterprise Institute.

If U.S. centuries are marked by defining changes, recent events combine to signal this new age:

Gay marriage: As the AIDS crisis spread across the U.S. in the mid-1980s, 55 percent of Americans said gay and lesbian relations between consenting adults should not be legal. Now, a public opinion tsunami has upended long-held views, shifting from 40 percent approval of same-sex marriage in 2009 to 60 percent.

When the Supreme Court ruled in favor of such unions, virtually no major political leader urged immediate action to overturn the decision.

Confederate flag: On June 22, the Indian-American governor of South Carolina, a black senator and a black congressman stood together to say that the Confederate flag should no longer fly at the State Capitol. It was an extraordinary scene in the state where the Civil War began.

Yet within days of the slaying of nine black Americans last month by a white gunman in Charleston, the Confederate battle flag’s time was up. GOP Gov. Nikki Haley stood with Rep. Jim Clyburn, a Democrat, and Sen. Tim Scott, a Republican, to say it should come down.

Immigration: A swell of movement from Latin America, and to a lesser degree from Asia, is changing it now. The new century will see whites become the minority within 30 years. The GOP presidential candidates include two Cuban-Americans, Sens. Ted Cruz of Texas and Marco Rubio of Florida.