Here’s my contention: that it is in middle school or junior high when youth develop reading tastes for specific kinds of stories, often known as genre. Young children learn to read, hopefully by 3rd grade. Soon after, they develop a love of reading itself. They know how to decode words, follow sentences, and make sense of a plot, but it is also in these late elementary school years they develop an appreciation for story. Then, comes a chicken and egg thing. Does the literature itself start to divide into genre, or is it kids that start to look for genre fiction and then the literature follows? No matter, but it is safe to say, young teens know not just that they like to read by 6th grade but also the specific reading interests with certain conventions, described as genre, which begin in their teen reading years and perhaps last into their adult life.
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There’s more, but I assume you’ve already stopped reading.