This study shows that bilingualism can delay the onset of symptoms of dementia by about 4-5 years compared with monolinguals. They still get the disease, but they're able to cope longer than monolinguals once they get it.
This article discusses whether becoming bilingual can delay the onset of Alzheimer's disease and dementia by up to 5 years. It is believed that learning another language improves functions like attention and alertness. Since a bilingual brain is constantly working, the brain becomes more resilient to the impairments caused by diseases. The article also mentions contradicting research that provides a different perspective on this topic.