At the same time, the 18-to-29-year-old respondents are more aligned with some progressive policies, though they have yet to fully embrace the label of "Democratic Socialist" applied to politicians such as Vermont Sen. Bernie Sanders or New York Democratic congressional candidate Alexandria Ocasio-Cortez.

Question: Why do they keep saying "Democratic Socialist" instead of "Social Democrat"?

Social democracy is a political, social and economic ideology that supports economic and social interventions to promote social justice within the framework of a liberal democratic polity and capitalist economy.

Scandanavian countries are Social democracies. Democratic socialists are another species entirely.

Democratic socialism is a political philosophy that advocates political democracy alongside social ownership of the means of production with an emphasis on self-management and democratic management of economic institutions within a market socialist, participatory or decentralized planned economy.

Democratic socialists hold that capitalism is inherently incompatible with what they hold to be the democratic values of liberty, equality and solidarity; and that these ideals can only be achieved through the realization of a socialist society.

...Democratic socialism is further distinguished from social democracy on the basis that democratic socialists are committed to systemic transformation of the economy from capitalism to socialism, whereas social democracy is supportive of reforms to capitalism.

The two are apples and oranges. The Prime Minister of Denmark explained in a speech at Harvard in 2015:

Government, Danish Prime Minister Lars Løkke Rasmussen said that while he's flattered to see Denmark discussed in a widely watched US presidential debate he doesn't think the socialist shoe fits.

"I know that some people in the US associate the Nordic model with some sort of socialism," he said. "Therefore, I would like to make one thing clear. Denmark is far from a socialist planned economy. Denmark is a market economy."

In Rasmussen's view, "The Nordic model is an expanded welfare state which provides a high level of security to its citizens, but it is also a successful market economy with much freedom to pursue your dreams and live your life as you wish."

Here's an article on the matter, which says Sanders is promoting Social Democracy not Democratic Socialism. Sanders may have said both over the years, but if so, that's due to his confusion, which should be pointed out and corrected so reporters don't keep repeating it.