Matthew Poole and continuators on Psalms 127:4

4 As arrows are in the hand of a mighty man ; so are children of the youth .

When they are shot out of a bow by a man of great strength against his enemy, which are of great use and power both to offend the enemy, and to defend himself.

Children begotten in youth, as an husband or wife married in their youth are called an husband or wife of youth, Prov. 5. 18. Isa. 54. 6. Ioel. 1. 8. and as a son begotten in old age is called a son of old age, Gen. 37. 3. And these he prefers before other children in this point, partly because such are commonly more strong and vigorous than others; and partly because they live longest with their parents, and to their comfort and support, whereas children born in old age seldom come to any maturity of years before their parents death.