A son of thirty years is David in his being king; forty years he hath reigned;

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

David was a son of thirty years, when he had begun to reign, and he reigned for forty years.

The age of David is conclusive as to the fact that the earlier years of Saul's reign (during which Jonathan grew up to be a man) are passed over in silence, and that the events narrated from 1 Sam. 13 to the end of the book did not occupy more than 10 years. If David was 20 years old at the time he killed Goliath, four years in Saul's service, four years of wandering from place to place, one year and four months in the country of the Philistines, and a few months after Saul's death, would make up the 10 years necessary to bring him to the age of 30.

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, Luke 3:23,
and he reigned forty years; and six months, as appears by 2-Samuel 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 he fled six months from Absalom; the other is, that he was ill in Hebron so long, and therefore are not reckoned.
(a) Hieron. Trad. Hebrews. in lib. Reg. fol. 77. I.