If you will fear Yahweh, and serve him, and listen to his voice, and not rebel against the commandment of Yahweh, and both you and also the king who reigns over you are followers of Yahweh your God, [well]:
So you do remember that he started out his career extremely early. As soon as he was weaned he was a public figure. He was there in the place of worship. The people who would gather for worship saw this little boy year by year as he grew and as he developed. They recognized that God's hand was upon this young man. He just sort of naturally grew into the position of leadership and judge over Israel.