Just a year and a half into his first term, Mr. Biden is already more than a year older than Ronald Reagan was at the end of two terms. If he mounts another campaign in 2024, Mr. Biden would be asking the country to elect a leader who would be 86 at the end of his tenure, testing the outer boundaries of age and the presidency. Polls show many Americans consider Mr. Biden too old, and some Democratic strategists do not think he should run again.

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He often shuffles when he walks, and aides worry he will trip on a wire. He stumbles over words during public events, and they hold their breath to see if he makes it to the end without a gaffe....