Verse 19 declares that the Nephi who had been keeping the large plate record passed away. This appears to have come after a lifetime of perhaps nearing a hundred years. His son, Amos, keeps the record until he dies. Then his son, Amos (another son named for the father), has the record.
This gets us to two hundred years since the birth of Christ in Nephite years. It is an important milestone because it is halfway to the final destruction. It is the righteous half. That will all change in the next verses.
The division between righteousness and wickedness that fits precisely on the two-hundred-year boundary is strongly suggestive that Mormon is fitting history into a more symbolic package. The events certainly happened, but it takes longer to shift a whole society than the movement of a single year to the next. The events surely happened, but would have happened over time. The marking of the year 200 as the turning point is a symbolic use of numbers and time.