Albert Barnes Commentary 2 Samuel 2:10

"Ish-bosheth, Saul`s son, was forty years old when he began to reign over Israel, and he reigned two years. But the house of Judah followed David." — 2 Samuel 2:10 (ASV)

Forty ... two - The numbers presented here are somewhat strange. First, regarding the forty years. If we assume Ish-bosheth’s reign did not begin until five and a half years after Saul’s death—which must be the case if the two-year reign mentioned in the text is correct—it is startling to find that Saul’s younger son would have been thirty-five years old at his father’s death. This would mean he was born about three years before his father became king and was five years older than David, the close friend of his older brother Jonathan. Furthermore, the age of Jonathan’s son, Mephibosheth, who was five years old when his father died, suggests that his uncle Ish-bosheth should have been younger.

Second, regarding the two years. David reigned seven years in Hebron over Judah alone (compare 2 Samuel 2:11). Therefore, if the two-year reign for Ish-bosheth is correct, it implies one of two possibilities. Either five years passed between Ish-bosheth’s death and David’s anointing as king over all Israel, or a similar five-year interval occurred between Saul’s death and the start of Ish-bosheth’s reign.

Of these two scenarios, the latter is more probable, as it has the advantage of reducing Ish-bosheth’s age by five or six years. However, the narrative in 2 Samuel 3 and 2 Samuel 4:1–12, which describes the “long war,” the birth of David’s six sons, and Abner’s conspiracy and death, seems to imply a period longer than two years. If this is the case, both numbers in the text would need to be corrected.