With questions about his age and mental faculties already in the spotlight, US President Joe Biden’s disastrous performance in the presidential debate, filled with pauses, memory lapses and rambling speeches did nothing to allay concerns about his mental fitness for office.

If the President is re-elected in November, at almost 82, he would be the oldest US President in history. Yet at 78, Donald Trump is no spring chicken. If he was elected in November, he would also be the oldest person elected President – surpassing Mr Biden – if he wins back the White House.

Attacks on Mr Biden’s age have been a popular talking point within the right-wing circles since the 2018 presidential race, but following the debate, Democrats were also critical of his performance.

Many pointed towards small mistakes such as saying he had created “15,000 new jobs” instead of 15 million. Mr Biden also said he “beat Medicare”, a type of state-sponsored medical insurance for retired and disabled people in the US. The debate ended with the two men bickering about golf and though he is just three years younger, Mr Trump appeared the much more youthful of the pair.

But how old is too old to serve as leader of the free world? And what can voters expect from a president who could be 86 at the end of a second term?

Dr Lawrence Cunningham, an NHS GP, told i that though brain ageing might technically start much sooner, it starts to become apparent for most people in their forties and fifties.

“In my experience, this is the age range when many patients start reporting issues with memory, attention, and processing speed,” he explained.

Exactly how fast this process goes involves a complex interplay between genetic predispositions, nature, and environmental factors, nurture, according to the GP.

Dr Cunningham told i that some people carry genes that predispose them to neurodegenerative conditions like Alzheimer’s disease, which can accelerate brain ageing.

“But these genetic predispositions are not the sole determinants of how our brains age,” he said.

Brain injury specialist Natalie Mackenzie, who holds a degree in cognitive neuropsychology and runs The Brain Injury Therapist said there is no magical age at which a person’s brain starts to decline, and ageing is inevitable, regardless of the lifestyle you lead, “just like wrinkles”.

“It’s not a clear case of ‘oh I’m 80 now’, again, it depends on a combination of activity levels, socialisation, general health and genetics,” she said.

“Ageing of the brain is as inescapable as ageing of the body, you can slow it down but you can’t stop it,” said the therapist, who has spent the last two decades helping people recover from brain injuries.

“That being said, there is no magical number when brain ageing suddenly starts to become apparent, whether at 70 or 80”.

“The slowing down of some cognitive processes starts as soon as the mid-to-late 30s, and progresses slowly from there, at a different rate to everyone”

She said every brain is as “unique as a fingerprint”, many people will be sharp as a feather at an age when others might start to show obvious signs of decline.

“There is some research about ‘SuperAgers’ who are over 80 with the same super healthy brains as a younger person. So we know age-related decline is not a forgone conclusion,” she said. But ultimately, brain ageing is the type of field where you definitely have “outliers”, people that differ significantly from the norm.

“There are definitely cases of people going into their eighties with minimal signs of decline, and others will present in the early sixties.”

While she did not comment on Mr Biden specifically, Ms McKenzie said ageing brains may be less able to effectively deal with high stress environments.

“Your brain gradually gets ready for a simpler, easier life, as you enter your golden years.”

How to slow an ageing brain?

Both Ms Mackenzie and Dr Cunningham agreed that the best way to slow down cognitive decline in old age was to learn new skills and try new things.

“The most important thing is try and do something which you haven’t done before,” said Ms Mackenzie.

For pianists, this might be taking Spanish classes, while for a tone-deaf Spanish teacher, this might be learning piano.

“As people grow older they tend to become settled within their careers and with their hobbies, they’re not learning new things at nearly the same rate as they did in teens and twenties. The best thing depends on the person, but it will be something that brings you out of your comfort zone.”

Laura Hughes, a nutritionist who holds a masters degree in neuroscience said nutrition plays a significant role in preventing cognitive decline, and consuming Omega-3 fatty acids was important for good brain health in old age.

Ms Hughes recommends “brain foods” including oily fish such as mackerel, as well as whole grains such as barley and quinoa.

Dr Cunningham points towards the large body of evidence behind the value of puzzles like Sudoku and mental activities to brain health – but thinks that physical exercise is equally important

“Exercise increases blood flow to the brain and promotes the growth of new neural connections,” he told i. “In my practice, I have seen patients who maintain an active lifestyle generally exhibit better cognitive function than those who lead a sedentary life.”