Donald Trump has made 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.

They claim these clips, which are painstakingly edited together and sometimes distort reality, are proof that Biden cannot effectively run the United States.

"His brain is straight gnocchi at this point," senior Trump aide Jason Miller said Thursday.

Sharks and Sinatra

Amid all the virulent criticism, Trump's team is trying to make voters forget that he is just three and a half years younger than his November election adversary.

Trump will celebrate another trip around the sun in grand style on Friday night in Florida.

"Join us in celebrating the birthday of THE BEST PRESIDENT EVER," reads the invitation, asking guests to come dressed in the colors of -- what else? -- the American flag.

For Matthew Foster, a lecturer in government at American University, "there are some noticeable differences in terms of physical capabilities," noting that Biden's gait can be rather stiff at times. 

But Foster also points out that Trump's schedule is much lighter, with more time between public appearances, and when he does address supporters, he often descends into long, disjointed, stream-of-consciousness tirades.

"He's giving off the old, drunken uncle vibe at times when he is giving these speeches, and this might be a sign of age, that you become less able to control all of your impulses," Foster told AFP.

"And for somebody who's already so impulsive, this could go into unforeseen territory."

At a campaign event in Las Vegas on Sunday, the fiery Trump launched into a mind-boggling diatribe that ping-ponged from shark attacks to electric batteries to electrocutions -- earning a chortle from Democrats.

Team Biden called it "unhinged."

In recent months, Trump also confused the leaders of Turkey and Hungary, warned that the world was headed for a second -- not third -- world war, and called fictional serial killer Hannibal Lecter a "wonderful man."

'When I go bad'

Trump is no longer subject to the same routine and regular medical examinations that his rival must undergo as the occupant of the Oval Office.

As head of state, Biden must take a litany of medical tests for transparency purposes. The detailed results are made public by the White House physician.

In contrast, no information has been made available in years about the health of Trump, who is known to be a fan of fast food. 

In mid-November, the presumptive Republican candidate published a short letter from his doctor, which said he was in "excellent" health, and that he had even lost weight -- without saying how much.

At the start of the year, Trump told supporters that he had recently taken a cognitive exam and "aced" it.

"I'll let you know when I go bad; I really think I'll be able to tell you," he said.

Not the 'deciding factor'

Whoever wins the November 5 election will be the oldest American president to take the oath of office. 

But do voters really even care?

Foster is not so sure.

"The big question we have to ask ourselves is -- is anybody going to vote for Trump because he seems younger and more energetic?" he said, adding that he believes debates on issues such as the economy, abortion and immigration will mean more.

In a politically polarized nation, where the election could come down to just tens of thousands of votes, the age of the candidates "is not going to be the deciding factor," Foster said.

"The stakes will be seen as too high by the time election day comes."