21 “And even unto oldness and old age”. These are two terms for old age, and are distinguished by the Greeks. For the gravity succeeding youth has another name among the Greeks, and after that same gravity the last age coming on has another name; for πρεσβύτης signifies grave, and γ·ρων old. But because in the Latin language the distinction of these two terms holds not, both words implying old age are inserted, oldness and old age: but you know them to be two ages.
“You have taught me Your grace from my youth; and even until now;” after my youth, “I will tell forth Your wonderful works,” because You are with me in order that I may not die, who hast come in order that I may rise: “and even unto oldness and old age,” that is, even unto my last breath, unless with me You shall have been, there will not be any merit of mine; may Your grace always remain with me. Even one man would say this, you, he, I; but because this voice is that of a certain great Man, that is, of the Unity itself, for it is the voice of the Church; let us investigate the youth of the Church.
When Christ came, He was crucified, dead, rose again, called the Gentiles, they began to be converted, became Martyrs strong in Christ, there was shed faithful blood, there arose a harvest for the Church: this is Her youth. But seasons advancing let the Church confess, let Her say, “Even until now I will tell forth Your wonderful works.” Not only in youth, when Paul, when Peter, when the first Apostles told: even in advancing age I myself, that is, Your Unity, Your members, Your Body, “will tell forth Your marvellous works.”
What then? “And even unto oldness and old age,” I will tell forth Your wonderful works: even until the end of the world here shall be the Church. For if She were not to be here even unto the end of the world; to whom did the Lord say, “Behold, I am with you always, even unto the consummation of the world”? Why was it necessary that these things should be spoken in the Scriptures? Because there were to be enemies of the Christian Faith who would say, “for a short time are the Christians, hereafter they shall perish, and there shall come back idols, there shall come back that which was before.
How long shall be the Christians?” “Even unto oldness and old age:” that is, even unto the end of the world. When thou, miserable unbeliever, dost expect Christians to pass away, you are passing away yourself without Christians: and Christians even unto the end of the world shall endure; and as for you with your unbelief when you shall have ended your short life, with what face will you come forth to the Judge, whom while you were living you blasphemed? Therefore “from my youth, and even until now, and even unto oldness and old age, O Lord, forsake not me.”
It will not be, as mine enemies say, even for a time. “Forsake not me, until I tell forth Your arm to every generation that is yet to come.” And the Arm of the Lord has been revealed to whom? The Arm of the Lord is Christ. Do not then forsake me: let not them rejoice that say, “only for a set time the Christians are.” May there be persons to tell forth Your arm. To whom? “To every generation that is yet to come.” If then it be to every generation that is yet to come, it will be even unto the end of the world: for when the world is ended, no longer any generation will come on.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)