My native language is Finnish. There are plenty of dialects in Finland, and the official written form of the language is kind of an artificial synthesis of various dialect traits -- nobody really speaks written Finnish.
That is, except me when I was six... My parents would go to a school meeting and the teachers and other parents would be surprised -- "Oh, you don't speak in written language like your son?".
I guess I had been spending quite a bit more time with books rather than people.
Occasionally I'll use an unusual word without even thinking about it, and then later realize that I've never uttered that word aloud before. Ever. It's a pretty weird feeling.
We play a game where you think of words you've never said before and try use them in a sentence. (So if you don't know roughly what it means, it doesn't count.) It's fun because you try unsuccessfully to search for words with the weakest indices before eventually finding profitable seams by thinking of the crazy stuff you've read.
> I guess I had been spending quite a bit more time with books rather than people.
I've sometimes had the same problem. my native language is French, and I mostly learned English by myself with books, computers, and music. Sometimes when I spoke, it sounded more like something a character in a book would say than what a typical person would.
Exposure to more flesh-and-blook Anglophones has mostly fixed that, though.
My parents always spoke to me as an adult. I'll leave it to you to judge how well it turned out.