Check out these numbers:
Based on exit poll data, the percentage of the total Democratic primary/caucus turnout made up of voters under 30 declined in the ten of the eleven states where this could be measured (California didn’t have exit poll data in 2016, and Minnesota and Maryland were 2016 caucus states that also didn’t have exit polls). As you can see, many of the declines were quite large: 27% in South Carolina, 29% in Alabama, 25% in Texas, etc.
In case you’re wondering, based on national population data, there were probably more 18-29 year old voters in these states in 2020 than in 2016 (nationally there were about 5% more births in 1999-2002 than in 1986-1989, although of course birth and migration rates vary quite a bit by state).
Nationally speaking, Sanders was a relative political unknown in 2015; four years later he was one of the half dozen best-known politicians in America, and his message of revolutionary reform, or reformist revolution, seemed well suited to bring out young alienated Gen Y/Z first-time voters. (I actually saw this bumper sticker last week).
It hasn’t happened, to put it mildly. Note the explanation “young people don’t vote” doesn’t work here, because they did vote in large numbers for Sanders four years ago. Why isn’t it happening this time around? (I don’t know to what extent these percentages reflect lower absolute numbers of young voters, and/or higher numbers of older voters).
As tonight’s the night when Sanders’s campaign is likely to be declared deader than Marley by the punditeratti, the question seems particularly salient.