These are tough times for the world’s democrats. The easy democratic transitions are history, the remaining partial democracies are stalled, and the newest liberal democracies are faltering as they struggle to hold on to past reforms. Chaos in Iraq, the tightening grip of Vladimir Putin in Russia and Hugo Chávez in Venezuela, and the ability of China’s political elite to paint a veneer of international respectability on a deeply noncompetitive autocracy all seem to reinforce this gloomy picture.

But prevailing wisdom can be wrong. In fact, many developing countries could improve their chances of maintaining high levels of freedom if they would just—demographically speaking—“grow up.” Since the mid-1970s, countries with a high proportion of young people and very rapid growth of those entering their working years (ages 15 to 64) have been far less likely to maintain democratic gains than those with more “mature” populations. In other words, a country’s chances for meaningful democracy increase as its population ages. We can detect this pattern by tracking the proportion of 15- to 29-year-olds in the working-age population in states that, in recent decades, have achieved a truly liberal democracy (defined here as “free” in Freedom House’s country ratings). When the young-adult proportion dropped into the range between 36 and 42 percent, full democracies evolved without the political backsliding or military coups that had been so common in Asian and Latin American politics. Where high levels of democracy emerged well before the young-adult proportion declined, countries typically settled...