The correct answer to this open question is the following.
Although the question is incomplete and does not provide the list of options, we can say that the individual who was a commander in the army at 16 and king at 20, was Alexander the Great.
Yes, Alexander the Great, son of King Philip II, was an extraordinary military man that led his troops to impressive victories and great conquests. He was born in 356 BC, in Pella, Macedonia. At age 13, he was a disciple of the famous Greek philosopher Aristotle, who taught him and guided him to become a better man. He was 16 he led his army to defeat the Byzantine Empire troops. And in 336 BC, at age 20, Alexander became the king of Macedonia after the death of King Philip II.