1 Samuel 17:33
And Saul said to David, Thou art not able to go against this Philistine to fight with him: for thou art but a youth, and he a man of war from his youth. And Saul said to David, Thou art not able to go against this Philistine to fight with him: for thou art but a youth, and he a man of war from his youth.
The verse belongs to the confrontation between David, still a youthful shepherd, and Goliath, the Philistine champion and experienced warrior, contrasting David's youth and lack of military experience with Goliath's lifelong experience in battle. In the verse Saul judges David unable and unfit to go against Goliath to fight him, and tries to dissuade him, because David is but a youth while Goliath has been a man of war from his youth. David is moved by zeal to defend God's honor and his people.
David does not change his purpose when Saul objects, and his youth and lack of conventional battle experience do not prevent him from facing Goliath. He answers by recalling that the Lord delivered him from the lion and the bear, and by trusting that God will deliver him from the Philistine. His strength is therefore trust in God's help rather than human preparation or armor.
Saul permits David to go against the Philistine. David's later victory over the Philistine champion demonstrates that God-given faith and courage can overcome the limits of age and experience that Saul saw.