Bilingualism and cognition: The impact of age of acquisition, language use and proficiency in cognitively unimpaired older adults

Background: The positive effect of bilingualism, especially later in life, is typically attributed to its role as a cognitive reserve factor. However, evidence supporting such benefits remains inconclusive or appears limited to certain populations, tasks, or cognitive domains. Some of these inconsistencies may stem from the tendency to define bilingualism as a dichotomous variable, rather than considering the multiple factors that contribute to the bilingual experience. Our study investigates the impact of bilingualism on cognition in cognitively unimpaired older adults, focusing on age of language acquisition (AoA), proficiency, and usage throughout life. Method: Data from 2415 cognitively unimpaired individuals (aged 45–74) from the Alzheimer's and Families (ALFA) study were analysed. Cognitive assessments included the Mini-Mental State Examination to assess global cognition, semantic verbal fluency for semantic lexical retrieval, Memory Binding Test for verbal episodic memory, and WAIS-IV subtests for processing speed (Coding), visual-spatial reasoning (Visual Puzzles), non-verbal abstract reasoning (Matrix Reasoning), verbal short-term memory and attention (Digit Span Forward) and working memory (Digit Span Backward and Sequencing). We defined three groups based on AoA (Early/Late) and proficiency (High/Low) of Catalan: 1) Early High-Proficiency bilinguals (n=1559); 2) Late High-Proficiency bilinguals (n=537) and 3) Late Low-Proficiency bilinguals, primarily Spanish-dominant (n=319). Results: Analysis of Covariance controlling for age, sex, education, and cognitive reserve, revealed that both Early and Late High-Proficiency bilingual groups outperformed Late Low-Proficiency bilinguals in verbal semantic fluency and processing speed. Additionally, Early High-Proficiency bilinguals scored significantly higher in verbal short-term memory than both Late AoA groups. Conclusion: Our findings highlight that the cognitive effects of bilingualism in ageing are domain-specific and largely driven by high proficiency and active language use, rather than early acquisition alone. These results suggest that bilingualism serves as a significant contributor to cognitive reserve and highlight the importance of high-level L2 proficiency in promoting cognitively unimpaired cognitive ageing.