2 Samuel 5:4

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 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, 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.