Something happens in Mormon's annalistic style of writing that seems odd. While in the process of listing what happens seemingly year by year, Mormon notes that in the forty and eighth year that Helaman "had two sons" (Lehi and Nephi), and then he mentions that "they began to grow up" (Helaman 3:21). One gets the image of two young children, yet just 16 verses and 5 years later (in the fifty and third year) Mormon states that "Helaman died, and his eldest son Nephi began to reign in his stead." So one has to wonder why Mormon chose to mention Helaman having two sons and their "growing up" just 5 years previous to this, since in order to succeed Helaman and fill the judgment-seat, Nephi would have had to of been somewhat mature.
The resolution to this dilemma might be found in the phrase "they began to grow up unto the Lord." There is an implication of a covenant process here. Mormon had just barely explained in verse 20 that Helaman,
did observe to keep the statutes, and the judgments, and the commandments of God; and he did do that which was right in the sight of God continually; and he did walk after the ways of his father, insomuch that he did prosper in the land.
These words imply a process of covenant obedience, which Mormon relates to "grow[ing] up unto the Lord." [Alan C. Miner, Personal Notes]