I just returned from 12 days in Davos, London, and Brussels, where my goal was to encourage political leaders to raise the minimum age to 16 for opening or having social media accounts in their countries. This is the second of my four norms for a healthier childhood, laid out in my book, The Anxious Generation: How the Great Rewiring of Childhood Is Causing an Epidemic of Mental Illness. I met with leaders from Indonesia, France, the United Kingdom, and the European Union. Some have already acted decisively (Indonesia and France); the others are likely to do so. And just as I arrived home, Spain and the Netherlands announced that they would raise the age, too.

All of this happened less than two months after Australia enacted the world’s first nationwide age limit, which requires users to be 16 for opening or maintaining social-media accounts, and which puts the responsibility for enforcing the age limit on the platforms themselves.

The tide is turning, but I have been shocked by how quickly it is happening. Social media has been dominating kids’ attention for decades. Now, in the span of just a few weeks, the landscape has been transformed. What happened?

The cognitive psychologist Steven Pinker can help explain it. His most recent book, When Everyone Knows That Everyone Knows: Common Knowledge and the Mysteries of Money, Power, and Everyday Life, explores the massive social change that can occur when widespread private knowledge suddenly becomes public knowledge. For example: Many people may privately know that a dictator is brutal, or that an ideology is bankrupt, yet nothing changes for many years until something happens that lets everyone know that everyone else knows it too, and that everyone knows that everyone knows that everyone knows. Once that threshold is crossed, new forms of coordination become possible. Social movements ignite. Regimes and walls fall. Norms can change almost overnight...