That is true even if there is evidence of the pathology of the disease – including amyloid plaques and tau tangles.

Clinical neuroscientist Tommasso Ballarini at the German Centre for Neurodegenerative Diseases in Berlin, Germany, says several studies indicate there is a protective role for bilingualism or multilingualism when it comes to dementia and cognitive decline.

He has researched the impact of early-life bilingualism in later life. There are, he explains, a number of hypotheses as to why second-language learning might stave off dementia.

According to one, speaking two or more languages engages and trains cognitive control processes and the underlying neural networks.

The challenge and benefits of learning another language

Ballarini says speaking more than one language requires “a frequent switching between languages or controlling and suppressing the language that is not used at the moment”.

These tasks are not strictly language-related, he says. They require and promote cognitive agility in the switching to one language and suppression of another, so you don’t mix up your words.

Also, he says, bilingualism may promote neuroplasticity – “the ability of the brain to reorganise itself in response to experiences and learning”.

Several studies have shown that bilingualism can shape both the brain’s functional and structural architecture, making it more resilient to the changes associated with ageing and neurodegeneration and, in that way, it can help to build our cognitive reserve.

This is key in delaying or even preventing the clinical signs of dementia manifesting even in the presence of the pathology of Alzheimer’s.

Mendez agrees that bilingualism can reshape the brain, making it more resilient to the changes associated with ageing and neurodegeneration.

Dr John Grundy at Iowa State University in the United States agrees. The incidence rate of Alzheimer’s is about the same in people who speak one language as people who speak two, he says, “but bilinguals tend to be able to stave off … symptoms for longer”.

That is cognitive reserve at play. So even though Alzheimer’s might be present in the brain, the resilience of neural networks is reinforced by many things – reading, socialising, exercise, music and speaking more than one language.

‘Use it or lose it’: building ‘cognitive reserve’ keeps dementia at bay

Grundy, who co-authored a review of research on this, explains: “When you start to learn a new language,” he says, “it’s very taxing on the frontal lobes which are responsible for executive functions. Executive functions include things like switching between tasks and working memory.

“To cope with these demands, the brain works to maximise efficiency by strengthening connections with other more automated parts of the brain … With time, there’s a shift – or sharing – of load from the frontal lobes to the back and deeper parts of the brain.

“When you get older, and the frontal lobes begin to fail, if they have a good working relationship with other parts of the brain, it can work around deficiencies in the frontal lobes to compensate.”

Does it matter when you learn a language? The jury’s still out on that one. Grundy believes earlier is better and before five is optimum. But learning a new language at any age will be effective.

Caitlin Ware, a researcher at Broca Hospital in Paris, France, who studies bilingualism and brain health, suggests the age at which you learn another language is less important than how often you speak it.

In a 2017 study, published in Frontiers in Aging Neuroscience, Ware references studies that show that older adults do indeed have the capacity to learn a second language, and that they can relearn previously acquired words just as well as younger subjects even years after the language was originally learned.

Real-life learning the key to mastering a second language

Ballarini’s team investigated the links between language use at different life stages and the cognitive impact in older adulthood.

“What we observed is that only the language use in early and middle life stages was associated with the cognitive advantages later on. However, we didn’t have information on the age of acquisition of the second language, so we cannot make any statements about when it would be most useful to learn a second language.”

Like Ware, he believes that while the age of acquisition might play a role, what’s much more important “is the frequent use of the languages in daily life”.

At 16, my mother went to school in France to improve her schoolgirl French. When she arrived there, she told me once – long before her memories were lost to her dementia – nobody could understand a word she was saying. But a year later, she remembered, she was actually dreaming in French.

She didn’t keep it up, though, and slowly the language was almost lost to her. But even in dementia I was astonished that I could ask her a question, How are you, Mum? in (my poor schoolgirl) French, “Comment ça va, maman?” and she responded immediately, without deliberation and with confidence: “Je vais bien, merci” [I am doing well, thanks].

I wondered often afterwards if she had just kept it up, kept using this second language, whether she would have reinforced her cerebral scaffolding.

Curiosity and the acquisition of knowledge is always going to be good for the brain. Learning a new language is just one more way to do this – building words, worlds, connections, and, it turns out, even your brain.