Story books picture David as a child when he faced Goliath. Is that fable or fact?
David was the youngest of eight sons.
He was too insignificant to be considered when the prophet Samuel came to anoint a man to become the new king of Israel (1 Samuel 16:11). He was in the field, caring for sheep, which even a child could do.
Anointed but without any means to make himself king, David returned to shepherding and practiced slinging stones.
Long before Goliath, David was recognized as a valiant man.
He was  skillful in battle, and an excellent musician (1 Samuel 16:18). So he was appointed to play his harp for King Saul.
Many years later, Saul didn’t recognize David (1 Samuel 17:55).
When David told how God had directed his aim in killing a lion and a bear (1 Samuel 17:34), Saul didn’t recognize him as the harp player son of Jesse.
Saul made mistakes in his reign, but he wasn’t a complete idiot. He would never have risked his nation to make them servants to the Philistines (1 Samuel 17:9) without being convinced that David would win.
David was a fully mature man.
Since Saul was head and shoulders taller than other men (1 Samuel 9:2), David must have been of full adult stature to be given Saul’s armor to wear (1 Samuel 17:38).
If David was six feet tall, he was still three feet shorter than Goliath. He had no hope to win the battle as a man. But victory was certain as he approached the giant as a child of God, with the power and authority of the Lord (1 Samuel 17:45).
David wasn’t a proven warrior.
He wasn’t a veteran in Saul’s army, but he was a man who had grown to understand the power that comes from surrender to the Almighty. He saw the giant fall to the ground, because he was God’s child of stature.
In all situations, no matter how little or how much I have, I must depend on Christ for my strength and guidance. — Philippians 4:13