According to a growing body of research, bilingualism could help counteract cognitive and brain aging, as well as help delay the onset of dementia. Indeed, several studies have reported that the cognitive performance of older adult bilinguals is better than that of their monolingual age peers, especially in executive functions and cognitive control tasks (Bialystok et al., 2009; Estanga et al., 2017; Ong et al., 2017; Ware et al., 2020). Additionally, retrospective studies have shown that the onset of dementia in bilinguals compared to monolinguals was delayed by up to 5 years (Alladi et al., 2013; Bialystok et al., 2007). However, prospective longitudinal studies have provided mixed findings on the putative protective effect of bilingualism on cognition (Mukadam et al., 2017), thus sparking a vivid debate in the research community. For example, Lawton et al. (2015) followed for 10 years community dwelling individuals and reported that bilinguals and monolinguals did not differ in the age of dementia onset (Lawton et al., 2015). Hence, the extent of the “bilingualism advantage” on cognition in older adulthood remains unclear and needs further clarification (Antoniou, 2019).

Bilingualism is typically defined as the frequent, daily use of a second language (i.e., L2). Indeed, the advantages of being bilingual do not stem from the mere knowledge of L2 vocabulary and rules, but rather from appropriate and frequent switching between languages, which demands a high cognitive control to inhibit potential interferences between languages (Calabria et al., 2018; Green and Abutalebi, 2013). Previous studies have shown that bilingualism could confer advantages on cognition, leading to an increased performance especially in executive control and attention tasks (Bialystok, 2017; Costa et al., 2008; DeLuca et al., 2020). At the neural level, the handling of 2 languages engages a network of brain regions that supports language control (Abutalebi and Green, 2016; DeLuca et al., 2020). According to Abutalebi and Green (2016), the language control network encompasses both cortical and subcortical brain structures, including the anterior cingulate cortex, pre-supplementary motor area, bilateral prefrontal and inferior parietal cortex, basal ganglia, thalamus and cerebellum (Abutalebi and Green, 2016). Older adult bilinguals show preserved neural resources within the cognitive control network, which might explain the cognitive advantage conferred by bilingualism (Abutalebi et al., 2014; Olsen et al., 2015). Indeed, cognitive advantages associated with being bilingual might be due to its role as a protective factor against age-related brain decline (brain maintenance) or by its recruitment of compensatory neural resources that have been shaped by bilingualism (Perani and Abutalebi, 2015).

In the present study, we investigated the effects of bilingualism in a large sample (N>700) from the DZNE-Longitudinal Cognitive Impairment and Dementia study (DELCODE) (Jessen et al., 2018). The DELCODE study includes in-depth cognitive assessments and brain MRI. Specifically, we aimed to test whether cognition and brain structural integrity in older adulthood are influenced by (i) being bilingual at different life stages, that is lifelong or early, middle and late life stages, or by (ii) the cumulative effect of bilingualism across life. Those are unsolved questions with relevant implications for the understanding of the putative protective effect of bilingualism on brain and cognition.