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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.
It's not an official number, but it could make a good start point. In the same time, around 69 000 books are published each year in France (according to registration of copyright, here). If we keep the same percentage, for 250 000 books published each year, between 1 800 and 3 600 are books from new authors. I know they're is a lot of weaknesses in my calculation: the first number come from a marketing announcement and doesn't include self-publishing when the second one include them, so the true number is probably higher.