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It's okay to publish a book at any age. If it is a good book (or at least good enough.) Worse come to worse, if you can't publish at age 13, you can try again at age 18 or age 23. A famous writer, Pearl S. Buck published her first book at age 10, I believe.
1 (Most picture books are for 3-10 year olds.)  Create your writing time and mullâabout what to write, about how to write it, about how to be daring. Among your key resources are your love of stories and language, your spirit, imagination and persistence.