When Mormon was selected to lead the military system at the age of sixteen, he had certainly been prepared and positioned by several means to assume this position. Looking mature and being responsible were good foundations, but he had to be groomed and trained for the role.
As he explained in Mormon 1:5, Mormon was a direct descendant of Nephi, which, according to the book of Jacob, meant that he had the right to be a king or leader. That right belonged to the direct descendants of Nephi. This may explain how he could have been selected at such a young age to fulfill that role.
Likely, his father was a military leader, and he trained Mormon. John Tvedtnes has written a very persuasive argument that Mormon may well have come from a military caste or line of Nephites, and that he learned certain things naturally from his father. In that day, people typically took over the trade of their fathers. Mormon’s father may very well have been in that line of work, with great responsibility and political influence. For purposes of comparison, other great military leaders in history assumed leadership roles at young ages:
Alexander the Great, officially known as Alexander III, was the son of Philip II, King of Macedonia. He had been tutored by Aristotle, and had opportunity for the finest education. Aristotle gave Alexander, who was very interested in history, a copy of Homer’s Iliad, the great story of the Trojan War. As any good, young Greek would, he probably knew that whole text by memory, so he knew the great military history of the Greek people. Mormon, too, had an interest in history, especially as he was destined to edit the old records of his people. He was learning of the history of the Nephite armies and their wars.
Alexander the Great fought alongside his father for several years, and successfully led the military and defended the nation while his father was away on forays. Then he defeated the Persian Empire, a huge empire, when he was only 23 years old. He had been considered very precocious at the age of ten, when he tamed his own huge, white Macedonian horse. In his day, people assumed that only someone with divine blood could possibly have done that.
Julius Caesar was born into an influential family, well educated, and entered military service at age 19. By age 26 his extraordinary career as a general and politician had begun. When Julius Caesar was assassinated, Augustus Caesar became his heir at the age of 19, inheriting two-thirds of the very large estate that Julius Caesar had amassed. Augustus led the battle at the age of 32, when he defeated Marc Anthony to become the sole ruler of the Mediterranean world and Roman Empire. He developed the state into an empire, in which, despite appearances, he was the sole ruler, and he extended and defended the outer borders with great success.
The young Louis became nominal king at the age of five. However, he did not rule personally until he was sixteen, at which time, he took over complete, singlehanded rule. He reformed much of the French political and military system with a strong and highly organized hand.
Thus, it is perfectly feasible for a precocious and serious-minded young man like Mormon, particularly one who had been both trained by his father and called of God, to become an important leader at a young age. There are several examples throughout history. In contrast to these historic rulers however, young leaders such as Mormon, David the shepherd boy/king, and Joseph Smith were not only very talented and precocious, but more than that they were called by God and led through the Holy Ghost.
Book of Mormon Central, “What Do We Know about Mormon’s Upbringing? (Mormon 1:2),” KnoWhy 226 (November 8, 2016).