Historians believe that David was born in Bethlehem sometime between 1040-1030 B.C.
David was probably around 10-13 years old when Samuel anointed him as the next king of Israel.
David was probably 15-17 years old when he slays Goliath. His youth is probably one of the reasons that Saul delayed giving his daughter to David as a wife like he had promised to the man who would kill Goliath.
Jonathan was much older than David.
King David made his sons Royal Advisers in Jerusalem when they were probably between the ages of 14-18. This apparently caused them to become prideful judging by their actions.
King David is one of the most amazing Biblical examples of God's grace. Though he sinned many, many times even murdering the husband of the woman he took in adultery, yet because of his repentant heart, God called him a man after God's own heart. (I Sam 13:14, Acts 13:22)