According to 1 Samuel 13:1, in the oldest Hebrew manuscripts, Saul ruled for two years (assuming Saul actually existed at all, for which there is no hard evidence). However, various [Christian] translators assume this 'must' be an error because Acts 13:21 says Saul reigned for forty years. Saul's age at 1 Samuel 13:1 is conveniently missing from the oldest manuscripts. (Josephus twice gives the length as 20 years, thouge one instance is altered to add up to 40 in many translations.)
1 Samuel 8:1 says that before Saul became king, Samuel was already old, and had adult sons who were to be appointed as judges, but they were deficient and there was outcry for a king to be appointed instead. So Samuel, in a hurry because of his already advanced age, anoints Saul as Israel's first king. Before Saul becomes king, Samuel is "old and gray" (1 Samuel 12:2)
Shortly after Saul becomes king, he appoints various military positions, including his son Jonathan, who is implied to be at least 20 years old (Numbers 1:3). Saul soon offers an illegitate 'burnt offering' and is 'rejected' as king of Israel.
According to 2 Samuel 4:2, David becomes king when he was 30 years old, and therefore wasn't even born until Saul had been king for ten years. Yet Samuel, who was already "old and gray" when Saul became king, apparently was in no hurry to appoint a replacement king after Saul was rejected. By the time the even older and grayer Samuel gets around to secretly anointing David, Saul had supposedly been king for about twenty-five years. And even then David doesn't actually become king for another 15 years after that, during some of which time a very very old Samuel goes into hiding with David (1 Samuel 19:18).
Before David becomes king, Saul supposedly commits suicide (1 Samuel 31:4) and he is killed by an Amelekite (2 Samuel 1:10). If we are to believe Saul reigned for 40 years, and we assume Saul was as young as 15 when Jonathan was born, Saul would have to be in his mid 70s when he died on the battlefield.
On the other hand, if 1 Samuel 13:1 is accepted for what it actually says, Saul actually only ruled for two years (assuming Saul actually existed at all), and would probably be in his 40s when he died. This would also make Jonathan much closer to David's age, likely with less than 10 years difference (with Jonathan more likely being the younger).