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.
Your mention of the sanctification of the priests reminded me of the first time that I realized *how* we as believers become a royal priesthood – I had never before spotted the reference to the priests being consecrated by eating the things with which atonement had been made for them (Exodus 29:33), and it was then that I realized that this, as well as the Passover, was part of what the Lord was doing at the Last Supper – He was consecrating the disciples as priests. (I believe that the prayers prior to sharing the bread and wine simply set them aside for holy use – it is when we receive them with faith that the Lord ministers to us).