David was barely a teenager when he fought Goliath is a Myth, although he is shown in many illustrations as very young. That comes from the Latin text which calls David a child, a miss-translation of the Hebrew, which calls him a young man. David was at least twenty-seven, maybe twenty-eight when he fought Goliath. The key to David’s age the two years for Saul’s reign and seven and a half years for David as king over Judah while he was in Hebron. David was thirty when he began his reign in Hebron, which came immediately after Saul’s two year reign. See 1 Samuel 13:1.