How old was David when Samuel anointed him king (1 Samuel 16:13)?
The passage that you refer to is as follows:
Thus Jesse made seven of his sons pass before Samuel. But Samuel said to Jesse, “The LORD has not chosen these.” And Samuel said to Jesse, “Are these all the children?” And he said, “There remains yet the youngest, and behold, he is tending the sheep.” Then Samuel said to Jesse, “Send and bring him; for we will not sit down until he comes here.” So he sent and brought him in. Now he was ruddy, with beautiful eyes and a handsome appearance. And the LORD said, “Arise, anoint him; for this is he.” Then Samuel took the horn of oil and anointed him in the midst of his brothers; and the Spirit of the LORD came mightily upon David from that day forward. And Samuel arose and went to Ramah. 1 Sam 16:10-13 (NAS95S)
Here we are told that Jesse made his seven sons come before the prophet Samuel one-by-one. But none of the sons was the one that God wanted to be king over Israel, except for Jesse’s youngest son David. God had rejected all of the others. God was looking at each man’s heart. He was looking for a man after His own heart. Finally, Samuel discovered that Jesse had one more son, his youngest. When this youngest son came before Samuel, the prophet anointed him with oil as the future king. This occurred at about 1030 B.C. How old was Jesse’s youngest son when he was actually anointed as king?
According to 2 Samuel 5:4, David was thirty years old when he actually became king in 1025 B.C.
David was thirty years old when he became king, and he reigned forty years. (NASB) 2 Samuel 5:4
It is estimated that David was twenty-five years old when he was anointed as the future king and thirty years old when he was finally anointed as the king. David would have been called an adult today, yet he was submissive to his father even at the age of twenty-five years. There is a great lesson here for us. God wants us to respect older people, especially our parents. Listen to Leviticus.
You shall rise up before the grayheaded, and honor the aged, and you shall revere your God; I am the LORD. (NASB) Leviticus 19:32
So David was twenty-five years old when he was pre-anointed as king over Israel and thirty years old when he was actually anointed as king.