"Do not cast me away when I am old; do not forsake me when my strength is gone." Psalm 71:9
I am drawing nearer and nearer to the season which the Psalmist either expected or felt. Many reasons teach the aged believer the need of this prayer. As his graces are still imperfect, so his powers are feelingly upon the decline. It was but little he could do at his best--and now less and less.
He feels other props and comforts dropping off apace. When he was young he had warm spirits and pleasing prospects; but now what a change of the friends in which he once delighted! In some he has found inconstancy--they have forsaken and forgotten him; and others have been successively taken away by death. They have fallen like the leaves in autumn--and now he stands almost a naked trunk. If any yet remain, he is expecting to lose them likewise--unless he is first taken from them.
Old age abates, and gradually destroys, the relish of such earthly comforts as might be otherwise enjoyed. Pains, infirmities, loss of sleep, the failure of sight and hearing, and all the senses--are harbingers, like Job's messengers, arriving in close succession to tell him that death is upon his progress, and not far distant!
If youth has no security against death--then old age has no possibility of escaping the grim monster. But though friends fail, cisterns burst, gourds wither, strength declines, and death advances--if God does not forsake me--then all is well.
"Even to your old age and gray hairs--I am He who will sustain you. I have made you and I will carry you; I will sustain you and I will rescue you!" Isaiah 46:4 - Letters of John Newton

Are we to think that God has failed? But the failure of a
Christianity that expresses what we have made of revelation
does not change at all what God has accomplished. He became
incarnate. Jesus Christ, the Son, died (and our sins were
pardoned). He is risen (and death, chaos, and the devil are
defeated). No matter what may be the mischances of history or
the errors and aberrations of the human race, these things
endure. What is done is done. Irrespective of what we make of
Christianity, God's work and accomplishment are complete, and
they are inscribed in human history.- Jacques Ellul (1912-1994), The Subversion of Christianity, Wm. B. Eerdmans Publishing, 1986, p. 12