Maybe it's just because I was brought up differently. I guess not everyone's parents read to them anymore, but mine did. Once I was moving out of the picture book stage, there were also other books that I owned or enjoyed reading. I got into the Nancy Drew books in fourth grade and read through (them slowly purchased) the Little House series in fifth grade. But it wasn't until maybe six grade that I even considered calling myself "a reader."
There have only been a couple of periods (school reading excepted) that I have devoured books. And the term "reader" seems like it has to apply to people who devour books (goodness knows I've met such people). I have this impression because, to me, reading is just something I do. I may read a particular book in a week or less, or I may go a couple of months without really reading much of any book (school reading excepted again). That doesn't seem like a "reader's" behavior. Yet I am a reader.
And I can't say I had that moment or that particular book that opened up the world of books for me. Sure, I can track my reading depending on what I was enjoying most at a particular time. (More or less in this order: Nancy Drew, Little House on the Prairie, Anne of Green Gables, Little Women, The Chronicles of Narnia, Victorian novels including Jane Eyre, The Lord of the Rings, Jeannette Oke and Liz Curtis Higgs, more C.S. Lewis and Tolkien, more Victorian novels, Twilight, a couple of other modern books, Harry Potter, Gayle Forman, etc.) But these were all stages in a greater picture. I can't say any of them opened up reading in general, just aspects of it. I like to think I learn from every book I read, even books I don't like: if I don't like it, I try to identify what about it I didn't like.
It's all a process, one that has no ending and one that, for me, never really had a tangible beginning, either.