This market resolves YES as long as Biden is still alive for the entire day of August 22 (CT), and hasn't been incapacitated in such a way as to render him incapable of performing his duties as president. If Biden dies or is incapacitated before then, it resolves NO.
August 22 is the last day of the Democratic National Convention, so this market can be used to hedge your bets on Biden's nomination.
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clarification: resolves no if he is in his current state?
@jim And also that if he steps down as democratic nominee, but not as president, does this then resolve as No? Or what if he steps down as democratic nominee, for specifically age-unrelated reasons (small chance)?
@Fedor If he's still acting president, then it resolves YES, aside from an edge case like, "He just got incapacitated and the Cabinet hasn't had time to invoke the 25th Amendment yet".
Stepping down for non-health/injury related reasons means he's still capable of acting as president and just doesn't want to. The point of this market is to explicitly exclude such reasons and only focus on the possibility that he isn't the nominee because he is physically uncapable of being the nominee.