In their battles against king Laman and the Lamanites, Zeniff "caused that all my old men that could bear arms, and also all my young men that were able to bear arms, should gather themselves together to go to battle against the Lamanites; and I did place them in their ranks, every man according to his age" (Mosiah 10:9). According to Hugh Nibley, Zeniff's troops were mobilized and organized according to the pattern of Israel, as we read in the Milhamah Scroll, the Battle Scroll. By age is the easiest, the most obvious, and the most natural and workable way--people of the same age groups working together. So that's what he did; "and I did place them in their ranks, every man according to his age." so they went up to battle against the Lamanites, and they went up "in the strength of the Lord" (Mosiah 10:10). [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 2, p. 45]
“I Did Place Them in Their Ranks Every Man According to His Age”
Zeniff describes placing his men "in their ranks, every man according to his age" (Mosiah 10:9). According to Brant Gardner, the "ranks" would be the battle ranks, and would consist of multiple lines of warriors. In ancient hand to hand battle, the force of numbers organized could easily overwhelm numbers unorganized. Similarly, depth of lines provided for reinforcement of the lines as the fighters in the front fell (these tactics were polished by the Romans and adapted to military strategies up through at least the Revolutionary War). Mesoamerican warfare would be less stringent for tactics, but the conception of ranks was similar, with similar purpose.
The likely scenario was that there were multiple lines behind each other, and the lines increased in age, with the old men in the last row as the final defense rather than the initial brunt of the fighting. This uses the body of the old, man, but preserves their strength. The final "rank" in the military line was probably king Zeniff himself. [Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/] ~nahualli/ LDStopics/Mosiah/Mosiah10.htm, pp. 7-8]