Mormon was appointed to be the leader of the Nephite armies at the age of fifteen (Mormon 2:1-2). He says: "notwithstanding I being young, was large in stature; therefore the people of Nephi appointed me that I should be their leader, or the leader of their armies" (Mormon 2:1). One might wonder why being "large in stature" would have such an influence on the Nephites? After all, Mormon was just fifteen!
According to Jerry Ainsworth, it was common for Maya to select large people as their military leaders. The Nephites also likely knew of the unique family from which Mormon came. If Mormon came from a wealthy, aristocratic family, as it appears he did, then his travels throughout the land of Zarahemla would not have gone unnoticed. Certainly his family's name, lineage, and status would have been well known among the Nephites. It is conceivable that information about Mormon's calling as the record keeper became public, especially since he endeavored "to preach unto this people" (Mormon 1:16). The people would have understood that he had been "visited of the Lord, and [had tasted and knew of the goodness of Jesus" (Mormon 1:15). They undoubtedly knew of his superb schooling and of his noble and/or aristocratic lineage, as well as his spiritual roots. Because of this, Mormon would have been a perfect selection for such a military position. The Nephites had a long-standing custom of choosing someone with the gift of prophecy as a military commander (see 3 Nephi 3:19). [Jerry L. Ainsworth, The Lives and Travels of Mormon and Moroni, p. 158]
Note* Nephi also considered himself "large in stature" (1 Nephi 4:31). [Alan C. Miner, Personal Notes]