Years ago, the conservative activist Grover Norquist was the guest speaker in a class I teach at the University of Pennsylvania.

“Older people are the base of the Democratic Party,” he told the class of predominantly liberal Ivy League students. “Do you know what they do every day? Die.”

Mr. Norquist’s argument, that Democrats were on the short end of the generational political divide, is no longer valid. Americans of retirement age are reliably Republican, while young voters, in the Obama era, have flocked to Democrats.

The shift raises a few important questions for the presidential race: Can Hillary Clinton approach the level of enthusiasm that President Obama achieved with young voters? And can Donald J. Trump persuade older voters that he’s not a big risk?