My first purchase of a "grown-up" book at age 7 was
A Princess of Mars. I stole the money, maybe 2 or 3 dollars, from my grandmother's cash-in-the-jar secret stash and walked 10 blocks to the nearest bookstore, all by myself, in a big city. I was so proud.
Until the theft was discovered.
But I aspired to be a grown-up reader because, as mentioned by others above, in my early childhood I noticed my parents reading whenever they got the chance.