How old was David when he fought Goliath?

from the old testament

• By piecing together what we know of David's life and the few times his age is mentioned, we can infer a general idea of his likely age when he faced Goliath. David was thirty when he became king (2 Samuel 5:4).
• Prior to becoming king, David spent an unknown number of years being pursued by Saul (1 Samuel 19–31). While we don’t know how long that was, the events during that time suggest it was more than a few years, with scholars estimating at least a decade. That would have placed him around twenty years old when he began to flee.
• Some have suggested that David was thus twenty when he fought Goliath. However, the evidence seems to point to him being even younger to account for the time he served Saul after defeating Goliath, including rising in the ranks of Saul's "men of war" (1 Samuel 18:1–5).
• Additionally, clues in the Goliath account suggest he was likely in his teens. The account says that his three older brothers were in the battle (1 Samuel 17:13), which could imply that the other brothers were not yet considered old enough to join. Since David was the youngest brother, he would be the one least likely to be considered of military age.
• Another clue to support David being a teenager at that time was that Saul and Goliath mock him for being only a youth (1 Samuel 33, 42). This implies he was an unexpected presence on the battlefield, suggesting he was very young.
• Being the shepherd of the family also indicates that he needed to remain close to home, going out only to deliver food to his brothers (1 Samuel 17:15, 17–18). He was not old enough to go to battle.

from the new testament

• David's age when he fought Goliath is not mentioned in the New Testament.

implications for today

What must David have felt like as he was mocked for his youth and his "presumption" in going against Goliath? What must his brothers have thought when David defeated Goliath? But David realized something that the others evidently didn't: When God is on your side, you're invincible.

God had been preparing David for a unique role in salvation history: a righteous king through whom the Savior, Jesus, would be born (Matthew 1:1). That preparation to be the fearless fighter who stood before Goliath included facing lions and bears against his flock (1 Samuel 17:34–37). But even as David defeated the animals, he realized that it was God, not his own strength, who protected him and his sheep. God taught David that He was the protector.

Our calling is not the same as David's. He was unique. But we do have the same God working in us who worked in David. Just as He trained David, God is training you if you are a believer. It might not involve doing something humanly impressive like defeating a large enemy singlehandedly. But God is preparing you to live life in a way that honors Him and teaches others how great He is. Your “training manual” is the Bible. David wrote many of the Psalms, but he did so by being familiar with the rest of Scripture and allowing its theology to shape his thinking. Likewise, make reading and knowing God’s word a priority. As you do, it will transform your thinking (Romans 12:2) and everything you do in this life, no matter how mundane it feels to you, will result in eternal glory for God!