About how old was Nephi when he started writing his record?
As a historian Nephi leaves no record of specific dates of any occurrences or the specific ages of any of subjects of his narrative. With the exceptions of Mormon and Moroni, who gave some specific dates as they wrote their histories in the ending days of the Book of Mormon saga, we can only guess at the ages of the characters in the Book of Mormon.
As you know, Nephi was the fourth of six brothers. When Lehi and his family were expelled from Jerusalem and fled into the nearby wilderness their father, Lehi, instructed them to return to Jerusalem to obtain a very precious historical record from its custodian, Laban. Although Nephi was the youngest of the four brothers, the older two being quite rebellious during the whole episode, he took the lead and used much ingenuity and bravery in obtaining the plates–actually killing Laban in the process. So, how old would he have been at that time? It’s anybody’s guess. My guess would be probably somewhere between 16 and 20 years old.
It is recorded in the Book of Mormon that after they left Jerusalem they traveled in the wilderness for eight years. At the end of that period they arrived at the seashore. Book of Mormon historians suggest that they probably arrived at the Arabian Sea somewhere on the coast of Oman. There Nephi constructed a ship to take them to the Promised Land. To construct the ship he had to find ore, smelt it and make tools with which to work the timbers from which the ship was constructed.
Then occurred their voyage across the ocean and their landing on the western shores of the Americas. Their landing location is not known, neither has any archeological been positively identified with a Book of Mormon site. Nevertheless, having arrived in the Promised Land, it was necessary to establish themselves, then for Nephi to have discovered gold, smelted it from the ore and fashioned it into thin plates on which he could inscribe the record about which you ask. So, it appears to me that he could not have been a very young man when he began his record.
Further, the first record that he kept was a secular record and was called the Large Plates of Nephi. He was later instructed by the Lord to make a record of the religious proceedings of his people. These were called the Small Plates of Nephi. That sequence is revealed in the Book of Mormon, 1 Nephi 19:2
And I knew not at the time when I made them [the Large Plates] that I should be commanded of the Lord to make these plates [the Small Plates]; wherefore, the record of my father, and the genealogy of his fathers, and the more part of all our proceedings in the wilderness are engraven upon those first plates of which I have spoken; wherefore, the things which transpired before I made these plates are, of a truth, more particularly made mention upon the first plates.
It was from the Large Plates of Nephi that the Prophet Joseph Smith began his translation of the Book of Mormon into English. However, the first 116 pages of his handwritten text were stolen, and the Prophet Joseph was commanded by the Lord not to retranslate those same pages, but rather to take his record from the Small Plates of Nephi.
So, how old would he have been when he began to write the second record, the Small Plates of Nephi, that comprise the first 142 pages of the Book of Mormon?
How much time would have transpired from the time they left Jerusalem until they were ready to embark for the Promised Land? He would have left Jerusalem, we guess, at age 16-20; eight years in the wilderness, arriving at the seashore at age 24-28; finding the ore, making tools and constructing the ship–let’s guess four years–they would have began their sea voyage at age 28-32; the journey across the sea, establishing themselves in the new world, finding and preparing the ore for his sacred record–??. So, what would you guess? Probably around age 40 or so?
So at least we can say that he was not a very young man when he began to write the record that we have as The Book of Mormon.