The Nephites were not simply fighting a defensive war, they were fighting a war for their lives. Once again, the analogy to the later Aztec conflicts can provide us some light on the situation in the land of Zarahemla:
“It is widely agreed that in preindustrial populations virtually the entire male population capable of bearing arms took part in military affairs and that no one was exempt from war service among the Aztecs. The makeup of the Aztec army, moreover, has been estimated at 90 percent of the male population. Nonetheless, while virtually all males were mobilized in nonstate societies, in states, this was likely to be true only in emergencies or defensive actions. Estimates of 90 percent participation are instructive for the cities attacked by the Aztecs….” (Ross Hassig. Aztec Warfare. University of Oklahoma Press, 1988, p. 59).
The Nephite situation was clearly one of defense, and with the size of the Lamanite army to be arrayed against them, clearly an emergency. It is in this light that we should understand the entrance of the stripling warriors into the fray. They would not have been used in an offensive war, but in a critical defensive struggle, they were certainly among those capable of bearing arms.