I believe that most readers learn to read from childhood. It’s not so much that mom and dad (or mom + mom or dad + dad or grandma + sister or whatever configuration makes up the modern family these days) encourage the child to read but that the child actual had books in his/her life. My daughter point out that we have always had books just lying around, asking to be picked up and read (oh, lookie there! personification! which my son is currently learning about in fourth grade; he’s currently reading The Hunger Games). I was raised in a home like that, where you really didn’t need to go to the library (although we always went to the library anyhow).
Among my favorite books as a child were mysteries; I devoured the original Bobbsey Twins stories, along with Trixie Belden. I was never a Nancy Drew Girl, but I understood her appeal (along with the Hardy Boys). All these tales certainly shaped my love of reading, but the first “serious” literature I remember reading as a child was The Chronicles of Narnia. The seven novels that make up the series tell an allegory that is often considered heavy handed by adults. However, when I was reading these stories (I read them twice: once as written and once in chronological order, which is different from how C.S. Lewis wrote the Chronicles) all I saw was an enchanted world of talking animals and mythical beasts battling on the sides of good and evil.
It was the Harry Potter of an earlier era, and it encouraged more than one generation of kids to become readers.