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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.
Once a publisher decides a book is going to be published, everything has to be coordinated so that books get out into the market when the company said they would. At a publisher, a managing editor keeps track of what everyone is doing. If you are self-publishing, you'll have to fill that role.