Charles Ellicott Commentary Psalms 127:4

"As arrows in the hand of a mighty man, So are the children of youth." — Psalms 127:4 (ASV)

Children of the youth — that is, the offspring of an early marriage. Aquila, “sons of young and vigorous parents.” The young man, with his numerous family around him, is like the vigorous warrior with his quiver full of arrows.