As the final battle prepares, Mormon utters that fatal statement of statement from the Book of Mormon. He “began to be old.” This is the classic statement that the end of life is near, and that one is in the mode of making the preparations for transition (see ??? for more information on this phrase in the Book of Mormon.)
As part of the preparations for the end, Mormon has a separate set of plates that he gives to his son, Moroni. Although the text indicates that he gave the plates to Moroni, it is clear that Mormon writes on the plates again. It would appear that this statement refers to the plates after they are finished, rather than any exchange of the plates at this very time.
[therefore I made this record out of the plates of Nephi]: It is tempting to see this statement as an indication that part of the preparation for the final battle was the creation of what we know as the Book of Mormon. Certainly four years would have been sufficient for its creation, and the reprieve from war assured by the communiqué from the Lamanite king would have allowed Mormon the mental peace to be able to divert some of his attention from preparations for war to preparations for a future purpose.
We know that in the three hundred and forty fifth year, the plates of Nephi began in the hill Shim, and were removed by Mormon (Mormon 2:16-17). Mormon doesn’t tell us the extent of these plates, but it is probable that they were the current set of plates comprising the overall record known as the plates of Nephi, which had been a generic name for the official plate transmission line for a thousand years. It is therefore quite unlikely that Mormon would have retrieved, and carried with him, the full plates covering a thousand years of Nephite history. What Mormon does tell us of these plates suggests that they were precisely the regular records on which other record-keepers had written (see Mormon 2:18). Those plates were with Mormon, but were not the set of plates upon which the record we may read was written.
We next see the full collection of plates in the three hundred and sixty seventh year when Mormon retrieves the plates from the hill Shim because the land is being overrun by the Lamanite/Gadiantons, and he wants the full set of plates to be safe (see Mormon 4:23). After collecting the plates, Mormon relents and becomes once again the general of the people. This would seem to indicate that while the full set of plates were in his possession, he did not have massive amounts of leisure time in which to compose and write his text. It would not be surprising, however, for him to begin to read those plates so that when he did get the time to write his text, he would have the background information ready.
The plausible sequence of events would therefore have Mormon writing as a regular record-keeper for at least twenty two years. He would not have been able to write what we know of the Book of Mormon without access to the records in the hill Shim, and he does not obviously have them in his possession until the year three hundred and sixty seven.
From his year 367 to 380, he had the records in his possession, and had to read them and digest them. It is possible that he at least began to compose his outline during this period of time. Based on the statement in this verse, we would then see the actual redaction of the final text coming in the years between 380 and the beginning of the year 385. During that time, however, he did not finish the work before the final Nephite battle. Some unidentifiable portion, containing at least the very last chapter, of the record was written after the battle.