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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.
In some parts of the United States, a person must be 19 years old before he or she is a legal adult. In Scotland, a person can sign some types of contracts when he or she is 16 years old, but must be at least 18 to sign a contract to make a major purchase, such as buying a house or other property. To form a legally-binding contract, each party who is signing it must be able to understand what is being signed.