Somewhere between the ages of 3 and 4. I learned from a read-along program that my mother purchased for me (I think it was called Brite), where they would put various fairy tales and fables over the backdrop of a group of talking animals or something to spice it up for children. I was reading Dr. Seuss in first grade, The Hobbit in third grade, and by fifth grade I was the all-time leader in our school accelerated reader program, with something like 140 points when most other kids had 10-15 (you would need to read the book, and take a test on the book through a rudimentary computer program). Unfortunately, this led to my downfall in the public school system, as I was very interested in Physics, History, and Chemistry in my pre-teen/teen years. Through my obsession with reading every book in the school library (and a nice chunk of the science books at the public library), I quickly outdistanced all of the high school course material, and was left reading college-level texts on my own. Segue into me falling asleep from boredom in classes, into actively avoiding going to class at all. I skipped somewhere around 90 days of my sophomore year, a similar amount of my junior year, and only attended the last two weeks of my senior year (this was the first year of the implemented testing system, where you could pass a competency test and validate the entire course simply based upon your test score).
Ah, but I ramble on.