2 Samuel 5:4 (BSB)

David was thirty years old when he became king, and he reigned forty years.

From 2 Samuel 5. Also in the ESV.

Commentary on 2 Samuel 5:4

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on 2 Samuel 5:1-5: Here is, I. The humble address of all the tribes to David, beseeching him to take upon him the government (for they were now as sheep having no shepherd), and owning him for their king. Though David might by no means approve the murder of Ish-bosheth, yet he might improve the advantages he gained thereby, and accept the applications made to him thereupon.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on 2 Samuel 5:4: David was thirty years old when he began reign,.... Over Judah, which was the age of his antitype Christ, when he entered upon his public ministry, Luk 3:23, and he reigned forty years; and six months, as appears by Sa2 5:5; but the months are not mentioned, only the round number of years given: two reasons the Jews (a) give for this; the one, that...
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on 2 Samuel 5:4-5: 2Sa 5:4-5 The age of David when he began to reign is given here, viz., thirty years old; also the length of his reign, viz., seven years and a half at Hebron over Judah, and thirty-three years at Jerusalem over Israel and Judah. In the books of Chronicles these statements occur at the close of David’s reign (1Ch 29:27).