“They are 80-plus-year-olds with memory performance at least as good as individuals in their 50s to 60s,” she says, adding they have “outstanding episodic memory – the ability to recall everyday events and past personal experiences”.

To join her study, as Seiger did, their thinking and language skills, attention, and other cognitive abilities must be average or above, also.

Knowing people with Alzheimer’s disease, Seiger has long understood the importance of maintaining physical and cognitive health to avoid the disease. So he walks his dog regularly, looks after his wife and enjoys cerebral challenges – such as doing puzzles and Sudoku games.

Rogalski is an associate director of the Mesulam Centre for Cognitive Neurology and Alzheimer’s Disease at Northwestern University in Chicago, Illinois, and director of its SuperAging Research Programme.

She first “operationalised” the term superager 15 years ago. She describes superagers as a rare group with a combination of distinguishing characteristics: biologic, genetic, cellular and psychosocial.

Superagers are not required to have superior fitness or physiologic strength. They are an enormously diverse group: some require help walking while others lead exercise classes.

But the physiology they all share – whether in running shoes or a wheelchair – is that they appear to have “more youthful brain integrity”, Rogalski says.

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It’s as if “they are on a slower ageing trajectory. The cortex [outer layer of the brain] looks more like a 50- to 60-year-old’s brain than that of an average 80-year-old’s,” Rogalski says.

Their cortex is much thicker, she stresses, adding their brains just look younger. There is no thinning of the part of the brain called the anterior cingulate, which is responsible for focus and attention, which in itself helps to build memories.

Rogalski and her team began observing changes seen in MRI scans over time and comparing those with the brains of average 80-year-olds. What they found was that the cortex of the average person in their 80s thinned at a rate of two-and-a-half-times faster than that of the superagers.

There have been around 80 superagers in the study since the research project’s inception, the oldest being 104. Only about 10 per cent of people who apply to take part meet the criteria. Not all are highly educated; rather, they represent a broad spectrum.

It seems, she says, that a unifying characteristic is the way superagers face adversity: they seem to have resilience, an optimistic approach to life, looking for the best in it.

One goal of Rogalski’s superaging research is to identify factors that promote health span – because the key is not how long you live, but how long you live well. It also helps to determine potential ways to avoid developing Alzheimer’s disease.

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The programme helps to identify protective factors that contribute to much better memory performance in late life, including genetic, lifestyle and brain factors.

The presence of superagers, Rogalski says, provides an important opportunity to change expectations in ageing and decrease stigma, not just among other people but within ourselves.

Rather than saying “I’m too old for X, Y, Z”, a can-do attitude irrespective of age seems to have a rejuvenating effect.

Dr Becca Levy, a professor of public health and psychology at Yale University in the US, supports this in her book, Breaking the Age Code. She flags examples of ways to debunk negative age stereotypes, such as that older people have trouble learning new information.

“Age beliefs can contribute to brain health,” she says. In other words, if you think you can do something in old age, you probably can.

This is evidenced by superagers who are often on second or third careers and, if they aren’t working, are often volunteering, fully engaged with their community and enjoying strong relationships.

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The attention region deep in the brain is larger in superagers and is full of large neurons called spindle neurons or von Economo neurons (VENs), which are thought to play a role in social processing and awareness.

Rogalski says that in postmortem examination of the brains of superagers, there were four to five times the number of these neurons than in an average octogenarian.

A decade ago, Seiger was diagnosed with macular degeneration, damage to a part of the retina, usually caused by age, which may cloud vision. But advances in eye medicine enabled doctors to help him maintain his sight.

Now he is returning the favour: “Anything to help others, I will do,” he has been quoted as saying. “I am thrilled to be where I am in my health, and I hope to be here for a long, long time.”

He will help even after death: he has agreed to donate his brain for the superaging research.

Superaging tips from a brain health expert

Brain health researcher Dr Marc Milstein, an internationally recognised speaker on optimising brain health and author of The Age-Proof Brain, implements superagers’ habits into his own routine:

2. After a 10-minute walk outside, he eats a breakfast high in protein, which might be poached egg and avocado on toast, or oatmeal with berries and peanut butter.

3. On the school run, he chats to his kids about what they’re learning, to help him learn about things outside his own field.

4. He gets them to play their music, to help him keep current, he says.

7. He does a little pre-lunch workout. In the summer, he goes swimming, which has been shown to improve memory, cognitive function and energy.

8. Lunch is another high-protein meal – salmon, salad and quinoa. He prefers to eat it with company because our brain thrives on social connection.

9. If his energy flags during the slow hours of the afternoon, Milstein tucks into something healthy. No highly processed snacks, rather something sustaining and salt- and sugar-free, like nuts or a banana.

10. On the drive home from work, he might listen to a funny podcast or an amusing audiobook, to help him to unwind.

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11. He plays team sports – to maintain those social connections.

12. He enjoys quiet walks in the countryside where he can collect his thoughts.