The Ammonite oath was evidently binding only upon those who had personally enunciated it. It did not cross generations. (See commentary accompanying Alma 24:11.) While we do not know the year they took the oath, they did so before Ammonihah was sacked, which occurred in the eleventh year of the reign of the judges (Alma 16:1–3, 25:2). It is now the twenty-sixth year of the reign of the judges (Alma 56:9).
An unanswered question is how old these young men were. The record does not say whether children also took the oath. If they were too young to speak, it seems unlikely that they did. But taking the outside case, if the oldest of the young men were born after the oath, they would be fifteen. Men age twenty or more would not be considered “young,” since it would not be unusual for even seventeen-year-olds to marry and have families, based on typical data from the ancient world. I hypothesize that some would be as old as seventeen (born prior to the covenant, but too young to repeat the words of the oath), but most would be younger. For the story to be as miraculous as Mormon’s telling, I speculate that these soldiers ranged from perhaps twelve to fifteen.
It seems significant that these young men “called themselves Nephites.” Their parents are the people of Ammon and perhaps maintained some connection to their identity as converted Lamanites. Their sons, however, have spent most (or all) of their lives in their new home and their self-definition is not Lamanite, but Nephite.