There are eras in history, like the 1950s, when older people set the cultural and moral terms for the young. And there are eras, like the 1960s, when it’s the other way around.

The current decade has been in the latter mold. Its true beginning was Dec. 17, 2010, when a 26-year-old street vendor in Tunisia, Mohamed Bouazizi, set himself on fire, setting off protests that quickly toppled governments across the region. Now it approaches its end with the 16-year-old Swedish climate activist Greta Thunberg named Time’s Person of the Year.

In between, the decade has been fundamentally shaped by the technological creations of the young, in the form of social media and mobile apps; by the mass migrations of the young, from Africa and the Middle East to Europe and from Latin America to the U.S.; by the diseases of the (mostly) young, notably addiction and mental illness; and by the moral convictions of the young, from the #MeToo and Black Lives Matter movements in the U.S. to mass demonstrations from Cairo to Hong Kong.

Why and how did the young dominate the decade? Let’s narrow the focus to America.

Demography first. What history usually thinks of as “the sixties” (beginning around 1964 with the Civil Rights Act and the Gulf of Tonkin Resolution) coincided, in the United States, with the coming-of-age of the baby boomers, roughly 75 million strong. Our current decade coincides with the coming-of-age of millennials, another generation of about 80 million. More people, more power — or at least more influence. By comparison, my generation, the underwhelming Generation X, numbers only 65 million.