YA 一辭, 可能來自美國的書籍分類的定義!!

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children's books: Books for children. As defined by the book-publishing industry, children are generally readers aged 17 and younger; many houses adhere to a fine but firm editorial distinction between titles intended for younger readers (under 12) and young adults (generally aged 12 to 17).
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Other classifications include easy storybooks (for younger school children); middle-grade books (for elementary to junior high school students); and young adult (abbreviated YA, for readers through age 17).