From The Washington Post:
"Speaking two languages may keep the mind sharp longer than knowing only a single language, even in those who can’t read.
Scientists reviewed the records of 391 bilingual and 257 monolingual patients diagnosed with dementia between 2006 and 2012 at a clinic in Hyderabad, India. Patients who spoke two languages developed the first signs of dementia an average of 4.5 years later than those who spoke only one language."
Younger readers may not be worried yet about age-related dementia.
There is evidence that language study can also make you smarter long before you grow old. Learning to communicate and read in other languages improves a capacity of the brain known as executive function, which can improve your mental performance in various ways.