As Scott T. mentioned above, JS was much older than 14 when the BoM was published. Furthermore, according to his mother Lucy's biography, Joseph was a yarn-spinner and teller-of-tall-tales from an early age. He had many years to concoct his story. Add to that that he almost certainly had help from Sidney Ridgon and Oliver Cowdery, then it hardly seems remarkable at all that he produced the BoM.
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