Psalms 71:18

Now also when I am old and grayheaded, O God, forsake me not; until I have shewed thy strength unto [this] generation, [and] thy power to every one [that] is to come.

Ver. 18. Now also when I am old and gray headed] Now, that the plum tree is full of blooms, the map of age is figured in my forehead, the calendar of death appeareth in the furrows of my face; let me do nothing to spot my white head. Let me, with the sun, give greatest glimpse at the going down, and, with the rose, retain my sweetness, though I have lost my colour. See Psalms 71:9 .

And thy power to every one that is to come] Mirus fervor Davidis in celebranda bonitate Dei, saith Vatablus here. David would propagate God's praise to all posterity.