Samuel said to Saul, You have done foolishly; you have not kept the commandment of Yahweh your God, which he commanded you: for now would Yahweh have established your kingdom on Israel forever.
The first problem is Saul’s age when he began to reign. No other text of Scripture gives us his age at this time. The NASB translators have supplied "40" and the NIV and NET translators "30." The AV translators wrote, "Saul [was . . . years old]," leaving the number undefined.