Well, as my wife reminded me ( but I do remember! ) , we had a small bookshelf in our apartment when I was in grad school, and our daughter, still crawling, would ritualistically pull all the books off the shelf, and leaf through them, not knowing a thing about them or what they meant.
She became a teacher, and still is today.
( And, as a matter of fact, but beside the point, she’s the mother of two of our three grandchildren. )
I remember thumbing through an entire set of encyclopedias from A-Z, fascinated with the pictures of bugs, animals, machines, etc. Eventually I began to understand the words that went with the pictures.
By fourth grade, I was reading at a high school level. By that time, I was reading the dime store novels my parents kept around. That's how I learned about sex by the way, through the steamy novels my mother used to read. My father was big on crime stories, westerns and action series (like "The Executioner"). I read those as well.