Saul was [forty] years old when he began to reign; and when he had reigned two years over Israel,
That Saul indeed reigned forty years is the conclusion from a very reasonable deduction.   "2 Samuel 2:10 relates that Saul's son Ishbosheth succeeded him on the throne at the age of forty; and since Ishbosheth is not mentioned at all among the sons of Saul as they are recorded in  1 Samuel 14:49 (with the conclusion that Ishbosheth was born after Saul came to the throne); therefore Saul reigned forty years.  This writer accepts this as true because the Apostle Paul accepted this as the length of Saul's reign (Acts 13:21), as did Josephus.  "Saul reigned eighteen years while Samuel was alive and twenty-two years after Samuel's death."[4]  Our own conviction is that one statement from the Apostle Paul is of more value than a library of writings by uninspired men.