For Mormon to be selected to lead the Nephite armies at the age of 16 is remarkable. What is even more remarkable was that he was actually only 15 when this took place! The phrase "in my sixteenth year" often causes confusion. This is language which makes Mormon sound older than he really was. Mormon started his sixteenth year after his fifteenth birthday. He would not complete his sixteenth year until his sixteenth birthday. Therefore, if he was still "in his sixteenth year," he was 15 at the time. The same concept is found in Joseph Smith's declaration that, in the spring of 1820, he was in his fifteenth year (JS-H 1:7). We understand by this that he was actually 14 years old when visited by the Father and the Son.
“I Did Go Forth at the Head of an Army of the Nephites”
Sterling W. Sill
"If you think it an inspiration that a 16 year old boy could win the leadership of a great national army what would you think of a man between the ages of 65 and 74 who was still the best man among his entire people for this top position of leadership, and in those days the general marched at the head and not in the rear of his troops. (Mormon 6:11) It is one thing to shoot a guided missile at an enemy a thousand miles away, but it is quite another thing to meet the enemy face to face, and with a sword or a battle axe, take on all comers, old and young, on any basis they might choose to elect; and still be in there fighting at age 74. No weakling or coward survives a test like that. His leadership and great skill in battle must have been an inspiration to those fortunate companions in arms who were privileged to fight at his side" (quoted in Daniel Ludlow's A Companion to Your Study of the Book of Mormon, p.299)