How old was David when he became king?

Answer: 30 years old

David was thirty years old when he became king, reigning first over Judah in Hebron.

“David was thirty years old when he became king, and he reigned forty years. In Hebron he reigned over Judah seven years and six months, and in Jerusalem he reigned thirty-three years over all Israel and Judah.”— 2 Samuel 5:4-5 (BSB)