The research was carried out in a multilingual area of India. There, researchers analyzed the records of more than 600 patients to find out at what age dementia set in. Over half of the patients were multilingual. They discovered that the onset of dementia was much later for the multilingual patients. The results were not affected by factors like education level, gender or economic level.
The researchers theorize that bilingual activities require the brain to be in a state of attention. This intense brain activity somehow keeps dementia at bay. They are not sure if other similar intense brain activities such as learning a musical instrument or doing puzzles would have a similar effect.
I hope this gives you one more good reason to keep up your English studies.