Being bilingual is a more unusual condition than most people think. It means speaking both very naturally and capably, and I wouldn't call myself bilingual in anything. My German wife is, after fourteen years in the UK, as she speaks English not quite without unnatural structures or pronunciation once in a while, but completely naturally, and faster than most of us.
There was a peculiar phenomenon in the days of British India, when British children were cared for by native servants, and often learned the native languages simultaneously with English. A small child's language learning ability goes far beyond an adult's, and is quite different in its nature. They grew up truly bilingual, as children of mixed marriages often do, without even an awareness that they are speaking different languages with different parents. Then they were sent away to school in the UK, and came back assuming they could speak the language. But at least for a while, they found it had gone entirely. It was also something of a joke among Indian soldiers, that young British officers often addressed them in the female forms, which they had picked up in the course of their domestic arrangements.
Language learning doesn't necessarily stave off dementia. A point to remember is that most languages, other than the simplest, are more inflected than English. The verb forms are different for different people, the adjectives are masculine or female when the object isn't, the adjectives have to agree with gender or number, etc. etc. Some use a form of the noun to perform the functions we would do with prepositions, like "with", "by", "of" etc. Some of the European ones have a plural and a singular "you", and use the plural one when they want to be formal or polite, even if there is only one of you.
The result is that they are far more difficult to begin than English, which any market trader overseas can learn to jabber a little. But when you've got those things you've got them, and they mostly obey the rules. English starts easy, but you never get to the end of its complications.
I agree that it is difficult to learn a language well without human teachers, or at least the supplementing of self-teaching materials with human interaction. I teach college students in Saudi Arabia, and I tell them you can't build muscles by lifting featherweight plastic weights. Strain strengthens the mind, too. Still, there is no substitute for reading things you want to read. There is a sort of takeoff point in learning a language, where it sticks in the mind. I recently opened some of my old French university textbooks, and found that I had no trouble with some passages I distinctly remembered finding heavy going in my second year at university, which was 1969-70. But at that time I spoke much more fluently in Spanish, which I learned on a more conversational type of course. When I spent some time in Spain a few years back, though, I found far more of it had gone. The difference is that I had still been reading, not regularly but from time to time, in French.
