Actually, she amended, perhaps just Bernie himself.

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Mr. O’Rourke entered the Democratic primary race this past week with an aspirational pitch and a semi-improvisational tour of Iowa, with a Hawkeyes baseball cap and bilingual profanity on the stump, broadcasting his message of generational uplift and immediately thrusting age into the main currents of the 2020 race.

The contenders leading in initial polls, Mr. Sanders and former Vice President Joseph R. Biden Jr., will be 79 and 78 by Inauguration Day 2021. President Trump will be 74.

Yet as party activists begin to appraise the field, they are already grappling with whether to once again embrace a younger candidate who reflects the future or shrug off age and elevate a veteran politician who most clearly represents their simultaneous craving for undiluted liberalism and someone who can thwart Mr. Trump.

If history is a guide, Mr. O’Rourke and other Democrats betting on a youthful appeal — like Cory Booker, 49; Julián Castro, 44; or Pete Buttigieg and Tulsi Gabbard, both 37 — should have an advantage. Of the last five Democratic presidents, only Lyndon B. Johnson, who ascended to the job because of the assassination of a 46-year-old president, was older than 52 when he was first elected president. The roster of Democratic losers in modern times is littered with nominees who were neither young nor new to the political scene.