Other leaders were also young. The text reports that "Moroni yielded up the command of his armies into the hands of his son, whose name was Moronihah" (Alma 62:43) in the thirty-second year of the reign of the judges (see Alma 62:39). Moroni was twenty-five in the eighteenth year (Alma 43:3-4, 17) just fourteen years earlier. Even if we assume that Moronihah was born when Moroni was fifteen, Moronihah could not have been more than twenty-four when he took over command of all the armies.
On the one hand, mortality rates in the ancient world were significantly higher than they are now. So individuals simply had to take over responsibilities at an earlier age. On the other hand, there may have been a cultural factor at play as well.
Bernardino de Sahagun reports the custom among the Aztecs of sending young men to live in a "young men's house" (tepuchcali):
And when [he was] yet an untried youth, then they took him into the forest. They had him bear upon his back what they called logs of wood--perchance now only one, or, then, two. Thus they tested whether perhaps he might do well in war when, still an untried youth, they took him into battle. He only went to carry a shield upon his back.While Sahagun is writing about Aztecs, not Nephites, and about customs of a much later time, we do not know how far back the customs stretch. The custom, however, provides a plausible parallel for how a man could rise to be a commanding officer at an early age.
And when [he was] already a youth, if mature and prudent, if he was discreet in his talking, and especially if [he was] of good heart, then he was made a master of youths; he was named tiachcauh. And if he became valiant, if he reached manhood, then he was named ruler of youths (telpochtlato). He governed them all; he spoke for all the youths. If one [of them] sinned, this one judged him; he sentenced [the youths] and corrected them. He dealt justice.
And if he was brave, if he took four [captives] then he attained [the office of] commanding general, [or] chief. (Fray Bernardino de Sahagun, Florentine Codex 3, appendix 5, in Arthur J. O. Anderson and Charles E. Dibble, Florentine Codex [Santa Fe, NM: The School of American Research, 1952], 4:53.)