As Biden departs, Trump set to face questions over his age and acuity

With a younger rival to emerge, the focus is likely to turn to ex-president, 78, and his often rambling, confused speeches

With 78-year-old Donald Trump now certain to face a Democratic candidate younger than he is, the Republican could have the tables turned on him over the questions of age and mental agility that he often sidestepped while Joe Biden was his opponent.

The age gap between Trump and any of his likely Democrat opponents - Kamala Harris, 59; Michigan governor Gretchen Whitmer, 52; Pennsylvania's Josh Shapiro, 51 - could make him the sole focus of voters' desire for a generational handover of power.

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