This Bible contradiction is from the Skeptic's Annotated Bible.
1 Samuel 16:18 View context
Then answered one of the servants, and said, Behold, I have seen a son of Jesse the Beth-lehemite, [that is] cunning in playing, and a mighty valiant man, and a man of war, and prudent in matters, and a comely person, and the LORD [is] with him.
1 Samuel 17:33 View context
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.