Albert Barnes Commentary 2 Samuel 5:4

"David was thirty years old when he began to reign, and he reigned forty years." — 2 Samuel 5:4 (ASV)

The timeline of David's life provides conclusive proof that the earlier years of Saul’s reign—during which Jonathan grew up to be a man—are passed over in silence. It also shows that the events narrated from 1 Samuel 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, his 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.