I'm not sure I've even seen someone say WHY they are so certain it's "for kids." Mostly they seem to be implying that all fantasy is for kids, or it's not gritty enough to be "for adults," or there's not enough sex. But none of that holds up as a definition of "book for kids."
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I saw a bunch of people posting about how The Lord of the Rings is for little kids and not that good, and I had to x out of that fast before I started replying. 🤣
