It is not clear exactly how old David was at this point in time. When Saul referred to him as a boy, he was not drawing attention to his youth but rather his lack of military training and experience.1 Likewise, when David objected to marrying Saul’s daughter Merab, he did not mention his age either (see 1 Samuel 18:18).
33 And Saul said to David, “You are not able to go against this Philistine to fight with him, for you are but a youth, and he has been a man of war from his youth.”