Geography: A wilderness area is close to the city of Lehi-Nephi. Not only does Zeniff command that the women and children take refuge here, but Noah also flees the approaching armies into this wilderness area (Mosiah 19:9). The area is close enough that the women and children were able to make a last minute flight in Mosiah 19:9, and sufficiently “wild” that the area itself provided protection from the invading armies. In Sorenson’s geographic correlation, these would be mountainous areas to the west of Lehi-Nephi.
Social: It was suggested earlier that the shaved heads of the Lamanite armies might declare an intention of destructive war (not being captured, that is, conquer or be killed rather than be captured – see the comments following verse 8). Zeniff’s actions here would appear to indicate that he understood this to be a serious invasion threat as he removes the women and children from harms way, and arms the old men. Zeniff sees this army as much larger than his own, and apparently as one bent upon destruction (requiring the safety of the women and children).
Zeniff describes placing his men “in their ranks, every man according to his age.” 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.
In these ranks Zeniff placed his men according to age. How did he do this? There are at least two ways it could have been done. The first might be to place all of the old men in their own fighting rank. The other ranks would also be age-segregated. While this fits the description, it makes for a weak battle strategy. The more likely scenario is 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.