2 Samuel 5:4

Thirty years old. — This statement of the age and of the length of the reign of David (which is repeated in 1 Chronicles 29:26, at the end of the history of David’s life) shows us approximately the length of time since the combat with Goliath as some ten or twelve years. It also proves that the greater part of Saul’s reign is treated very briefly in 1 Samuel, and further shows that David was seventy years old at his death.