We only have one case to base this on though, no? Genie? That said, it’s important to teach deaf children sign language as early as possible, which I’m sure is much more deeply researched.

i believe this is the case, but there is probably external evidence that points to this as well. The sign language is an interesting point though.

I’ve always wondered if this idea may have some issues relating to biased sampling. We look at older people who either do or do not know multiple languages and determine whether they’ve begun to develop dementia-like symptoms and draw conclusions from there. Have we ruled out the possibility that those who are predisposed to dementia will struggle to become bilingual in the first place?

that's a possibility i suppose, but i believe the general field of science surrounding this also supports the idea that keeping your brain engaged and healthy improves your mental health as you age. Similarly to your physical body, which would make sense, though that technically isn't scientific lol.

There are likely studies that account for this as well, people who learned a second language later in their life, vs people who grew up bilingual and people who are monolingual.

It's also generally understood that your brain is highly plastic, and if you don't use it then you lose it. It's why applying learned concepts is so important, and it's part of the reason you need refreshers after a few years, so this also seems to line up with that idea.