And the sooner you pick up foreign speech, the greater the protective effect.
Experts found being bilingual helps build brain power.
These greater “cognitive reserves” mean a person is able to keep dementia at bay for longer.
Italian scientists carried out brain scans on 85 patients with Alzheimer’s disease.
Half the participants spoke two languages, while the rest only spoke one.
Scientists found bilingual patients were on average five years older at the same stage of the disease.
And scans revealed they had stronger connectivity in the decision making part of their brain.
Experts believe this slows the damage caused by disease.
Lead researcher Professor Daniela Perani said: “Overall, these findings strongly suggest that bilingual individuals with Alzheimer’s disease compensate better for the loss of brain structure and function.
“It suggests the earlier you learn another language and the more you use it, the greater the protective effect.”
The research is published in the journal the Proceedings of the National Academy of Sciences.
Dementia affects around 850,000 Brits, with Alzheimer’s responsible for the majority of cases.
And the number is forecast to hit one million in under a decade and two million by 2051.
There is no effective treatment, with current medications only helping to slow down the symptoms.
Dr Clare Walton, Research Manager at Alzheimer’s Society, said the study reveals how speaking an extra language protects the brain.
She said: “This elegant study provides new evidence that people who are fluent in more than one language have some protection against dementia.
“Brain scans showed that lifelong bilinguals have stronger connections between certain brain areas compared to those who only speak one language – this appears to allow their brains to cope better with damage before they start to show outward signs of dementia.”
And Dr David Reynolds, Chief Scientific Officer at Alzheimer’s Research UK, said bilingual adults have more “resilient” brains.
He said: “There is increasing evidence that being bilingual throughout life could increase our cognitive reserve.
“This study shows there are differences in the brains of people with Alzheimer’s who are fully bilingual that could explain why they are resilient to damage.
“The findings lend weight to the theory that keeping the brain active may provide a form of cognitive reserve, helping to delay the onset of symptoms as diseases like Alzheimer’s develop.”