Can learning a new language stave off dementia?

My father decided to start learning French when he was 57. He hired a tutor to meet with him twice a week and diligently completed his homework before every lesson. Before long, he was visiting the French bakery across town to practice his pronunciation (and buy macarons). Now, two decades later, he's on his third tutor.

On the surface, his retirement hobby seems a little random - our family has no connection to French-speaking countries - but his motivation ran deeper than a passion for pastries. My grandmother developed signs of Alzheimer's disease in her early 70s, and studies suggest that being bilingual can delay the onset of the condition by up to five years.

Drawn by that potential benefit, many people, like my father, have attempted to pick up a new language in adulthood. According to a survey conducted by the language learning app Memrise, 57% of users...