Saul was [forty] years old when he began to reign; and when he had reigned two years over Israel,
However in 1 Samuel 13:3 Saul’s son Jonathan is old enough to lead an invasion against a Philistine garrison. Jonathan must have been at least about 20 to do that. If he was about 20, and this was the beginning of Saul’s reign, we have two problems. First, Saul must have been somewhat older than 30 when he began ruling. Yet this would make him quite old when he died in battle, as explained above. I think he was probably about 40 even though this would make him about 80 when he died. Second, if Jonathan was about 20 at the beginning of Saul’s reign, he would have been about 60 when he died with Saul since Saul reigned about 40 years (Acts 13:21). If David was a contemporary of Jonathan, as 1 Samuel implies, David began reigning when Jonathan was about 60. Yet 2 Samuel 5:4 says David was 30 when he began to reign. In spite of the disparity in the ages of David and Jonathan, it seems that Jonathan was indeed about 20 or 30 years older than David. [Note: See Leslie McFall, "The Chronology of Saul and David," Journal of the Evangelical Theological Society 53:3 (September 2010):475-533.] 