So, in my books, I spell out nearly all numbers. I know it’s not in line with any style manuals, but I do it for a reason.
When I was a kid/tween/teen, I was reading a book and the character’s age was given as 9. This 9 year old girl went on to drink alcohol and drive a car in later chapters and it just didn’t make sense. What the hell? This is possibly the worst behaved 9 year old in the history of the world. Towards the end of the book, she gets pregnant. Holy crap. Also, she was in high school. Why was a 9 year old a senior in high school. I had checked it out from the school library.
After I finished the book, I returned it and next time I went to the public library I decided to ask the librarian about it. She grabbed the library copy, the character was 19 not 9. Well that changes things significantly! The book now made sense. For some reason the edition I read had a printing error and the 1 in front of the 9 was missing.
When I started experimenting with writing myself decades ago, that incident was still fresh in my mind and I got in the habit of writing the numbers out… because there is no way to confuse NINETEEN with NINE. Fiction style guides say to write out one through ten and use numerals to indicate everything else when writing… but I don’t because twenty-nine also can’t become 9 if I write them out. And so, I write out all numbers less than 100.