Daily use of a second language, especially when begun before the age of thirty, is consistently associated with better cognitive control and possibly later onset of dementia. The evidence is, however, associative, and at least one large study found no effect. No statements can be made about music on the basis of the available claims.
Multiple studies suggest that bilingualism can have a beneficial effect on the ageing brain. An experiment using the so-called Simon task (a test of cognitive interference) showed that bilinguals exercised better cognitive control than monolinguals, and that this advantage was greater in older than in middle-aged participants (PMID 15222822). A brain-imaging study added that age-related decline in cognitive control occurred in monolinguals but not in bilinguals, and that bilinguals showed less brain volume loss as they aged (PMID 29360516).
Regarding dementia and cognitive decline, the findings are mixed but predominantly positive. In an Indian community study (1,234 adults aged 60 and over), dementia was more than ten times rarer in bilinguals than in monolinguals (0.4% vs. 4.9%), and mild cognitive impairment was also less common (PMID 38376105). A review article describes that bilinguals typically develop dementia symptoms later, function better with comparable levels of brain deterioration, and show a specific pattern of faster decline in the final stages of dementia, which points to prolonged compensation (PMID 33771449). A literature review adds that trilingualism may offer even more protection than bilingualism alone (PMID 28895004).
The age at which a person becomes bilingual appears to matter. The DELCODE study (746 participants) found that bilingualism that began between the ages of 13 and 30 was associated with better performance on memory, working memory, executive functions, and language at older ages. Bilingualism that began later in life conferred no measurable benefit. Brain volume did not differ, but bilinguals appeared to use their brain volume more efficiently (PMID 36706574). A large European study (86,149 participants, 27 countries) found that countries with higher levels of multilingualism had less accelerated biological ageing, although multilingualism was measured at the country level rather than the individual level in that study, which complicates interpretation (PMID 41214212).
There are, however, also clear counterweights. The SALSA study followed 1,499 older Latino adults for up to 9 years and found no benefit whatsoever of bilingualism on cognitive decline or baseline scores compared with monolingual English speakers (PMID 28967765). Moreover, bilingualism also has a downside: bilinguals have, on average, a smaller vocabulary in each language and are slightly slower at word retrieval. Second-language learners who do not have fluent command of the language do not yet show cognitive advantages (PMID 28895004). Evidence regarding music is entirely absent from the studies provided: nothing can be said about that part of the question on the basis of the available claims.
In summary, there is a consistent pattern of associations showing that daily use of a second language goes together with better cognitive control, greater cognitive reserve, and possibly a later onset of dementia symptoms in old age. The evidence is, however, almost exclusively associative (no experimentally demonstrated causal relationship), and at least one large longitudinal study found no effect. Lifelong daily exposure to a second language appears to be the key factor, more so than incidental language use or late-life learning.
All claims are based on observational and cross-sectional studies plus literature reviews (PMIDs 15222822, 41214212, 36706574, 28967765, 28895004, 38376105, 33771449, 29360516). No randomised controlled trials are available. Music is not represented in the claims provided.