Donald Trump has made U.S. President Joe Biden's advancing age one of his main campaign rallying points, trying to position himself as an energetic alternative to the sometimes unsteady 81-year-old Democratic incumbent.

But the Republican former president is celebrating his 78th birthday on Friday, and also has shown signs of slowing down, both physically and mentally.

Nearly every day, the billionaire businessman's campaign team sends out videos of Biden stumbling or stuttering, looking worn-out or appearing disoriented at public events.