I give the side eye to much of the tone of these book descriptions. (For example: I am struck in bookshops by racks, newly erected in the last decade, offering "teen fiction". If walk-in, walk-through bookshops survive (not a certainty), I shall expect soon to see racks spring up labelled "old guys' novels". I believe the proper response to this is "fuck you".) HOWEVER, the listicle did inform me of a novel I am currently dying to get my hands on: a romance penned by a 10 year old in 1919 called The Young Visitors.
I can think of a couple other novels written by the very young. There's Eragon, of course, which was started by Paolini when he was 15, and it shows. My quip about this book: not so much standing on the shoulders of giants as having a piggy back ride. There's also Strange Evil by Jane Gaskell, written when she was 14. Strange Evil was included in a list by China Mieville of the 50 sci fi and fantasy books every socialist should read. I've read about half of Gaskell's book, and it's utterly charming and wholly bizarre.
Can you guys come up with others? I think I'm looking for books written by people well under the age of 20, not just teens in general. Frankenstein was written by the 19 year old Mary Shelley, for example, but I think I'd strike her from the list, as she was already married, plus the Victorians had a very different sense of the age of majority. Novelists generally don't get points for youthful precociousness -- not like musicians or poets, anyway, like Mozart or Keats -- but it's still cute when it happens.