A 2014 study in Frontiers in Psychology, led by Thomas Bak at the University of Edinburgh, found that young adults proficient in two languages did better on attention tests than those who spoke only one. They were better at ignoring irrelevant stimuli and focusing on relevant information, whether they had learned their second language in infancy, childhood or their teens.
The study was small, with 38 monolingual and 60 bilingual university students. Earlier work by the same group found that people in India who spoke more than one language developed dementia about four years later, on average, than those who spoke one.
via Learning a New Language at Any Age Helps the Brain | Live Science