Hen I was young I read the Book of Knowledge, cover to cover. Tried to do everything they suggested experiment wise etc. Went to the library checked out all the books. When I was in the third grade I had read all the 6th grade books. I did all of this because I wanted to. Not because anyone encouraged me to. When I was in high school they tested my IQ and it was very high. I attributed to reading all those books when I was young.
I was fortunate to grow up in a house full of books and with highly literate parents. My innate curiosity drove me to read almost everything I could get my hands on.
You were a literate child because you already had a higher IQ. The limits of IQ are baked in at birth, you can’t increase it, only maximize it by about 5 points.