The studies I've read on bilingualism state that it delays onset by a few years, not that it prevents it. Let me duck for a cite though... Here's one "Recent

The studies I've read on bilingualism state that it delays onset by a few years, not that it prevents it. Let me duck for a cite though... Here's one "Recent meta-analyses report that active bilingualism is related to later onset of symptoms and, thus, diagnosis of dementia by as much as 5–7 years relative to comparable monolinguals, despite brains in both cases accruing increased pathology similarly" https://pmc.ncbi.nlm.nih.gov/articles/PMC8847162/ https://pmc.ncbi.nlm.nih.gov/articles/PMC8847162/ It also notes.. "However, as outlined below, simply being bi- or multilingual it is not sufficient for protection against cognitive decline, otherwise, a clear majority of the world’s population would be equally protected (considering that more than half of the world population speaks more than one language; De Houwer, 2021). It is important to keep in mind that only certain types of so-called ′active ′ bilingualism will have the maximum effect upon brain health, i.e., those who are amply exposed to their languages, use them regularly and are otherwise highly engaged in contexts that require linguistic switching."