In this framing, Sanders is not offering his more youthful constituency a radically new contract. Instead, he is extending the terms of an existing social contract to cover more—and, necessarily, younger—Americans.

Social Security and Medicare were the product of a consensus that the economy had broken the process of growing old. Today’s progressives argue, in part, that the economy has broken the process of growing up.

In the early 1900s, half of America’s senior citizens lived in poverty and more than half were uninsured. Over the next few decades, the federal government passed several laws—most importantly Social Security, Medicare, and Medicaid—that had the effect of making old age more comfortable and equitable. The senior poverty rate fell from about 50 percent in the 1930s to less than 10 percent today. The expected life span for elderly Americans increased by several years.

It would be hyperbolic to suggest that young Americans face the similarly dire circumstances of an impoverished 70-year-old in 1931. Quite the opposite: Some will claim that, with the unemployment rate under 4 percent and a record-low share of Americans saying the economy is the country’s most important problem, the 2020 environment is just dandy.

But this general optimism conceals an important divide. In November, the Conference Board, a nongovernmental research organization, reported that the economic-confidence gap between consumers under 35 and those over 55 was the largest on record. For the past 40 years, younger Americans have historically been the most optimistic, but that relationship has dramatically inverted over the past five years. Some of that inversion surely reflects partisan differences, since young Americans are more liberal than the rest of the country. But political polarization doesn’t explain it all.

Derek Thompson: The Millennials-versus-Boomers fight divides the Democratic Party

The simple fact is that young people live and work in a very different economy from their parents. And one good way to see this is to walk, beat by beat, down the road to adulthood.

Start with college. Millennials are the most educated generation in U.S. history to date. They “did what they were told,” as Sanders once said. “They got an education and worked hard. But instead of being rewarded, Millennials are now being punished with crushing student debt.” According to a study commissioned by Sanders and written by the Government Accountability Office, 45 percent of Americans ages 25 to 34 have student loans, compared with just 16 percent of Baby Boomers at the same age. Millennials’ loan-to-income ratio is more than double that of previous generations.

Next, there’s the labor force. While the unemployment rate is low, wage growth has fallen behind its historical trend line, putting the American dream at risk. In 1970, a 30-year-old had a 92 percent chance of earning more than her parents did at the same age. But today, she has just a 50 percent chance of outearning her parents. In these 40 years, this version of the American dream has gone from a near-certainty to a coin flip. Meanwhile, the cost of many necessities has gone up. Medical coverage is expensive, even for the full-time employed, and child care is often unaffordable, even when both parents are working.