They shall still bring forth fruit in old age - As a tree that is carefully planted and cultivated may be expected to live long, and to bear fruit even when it is old. It is true that such a tree may be cut down; or that it may be blown down by winds and tempests; or that it may be unproductive, but as a general rule, and as laying the foundation of a reasonable hope, such a tree may be expected to live long, and to produce fruit even when it is old. So of one devoted early to God, and trained up under the influences of religion. The care, the culture, the habits of temperance, of industry, of moderation, and of sobriety so formed, are favorable to length of days, and lay the foundation for usefulness when old age comes. An aged man should be useful. He should feel that whatever wisdom he may possess as the result of long study and experience, belongs to God and to truth; that one great reason for sparing him is that he may be useful; that the world needs the benefit of his counsel and his prayers; that his life is lengthened out not for his own ease or enjoyment, but that virtue and piety may be extended in the world by all the influence which he can bring to bear upon it in advanced years. It may be added that, as a matter of fact, those who are thus trained and are thus preserved, are useful in old age. No one thus spared need be useless; perhaps almost none are. There is something appropriate for old men to do, as there is for the young and the middle-aged; and it should be the object of an aged Christian to find out what that is, and to do it. The word rendered "old age means literally grey or hoary hair."
They shall be fat - The meaning is, that they shall be vigorous, or have the appearance of vigor and health.
And flourishing - Margin, as in Hebrew, "green." This image is taken from a tree, as if it were still green in old age, or gave no indications of decay.

They shall still bring forth fruit in old age - They shall continue to grow in grace, and be fruitful to the end of their lives. It is a rare case to find a man in old age full of faith, love, and spiritual activity.

They shall still bring forth fruit in old (i) age; they shall be fat and flourishing;
(i) The children of God will have a power above nature and their age will bring forth fresh fruit.

They shall still bring forth fruit in old age,.... Being thus planted and watered, they shall not only bring forth the fruits of righteousness, but shall continue, and go on to do so, and even when they are grown old; contrary to all other trees, which, when old, cease bearing fruit; but so do not the righteous; grace is often in the greatest vigour when nature is decayed; witness Abraham, Job, David, Zachariah, and Elisabeth, and good old Simeon, who went to the grave like shocks of corn, fully ripe:
they shall be fat and flourishing; or "green", full of sap and moisture, abound with green leaves and precious fruit; or, in other words, abound in grace, and be fruitful in every good work: being ingrafted into the true olive, the church of God, they partake of the root and fatness of it; having a place in the house of the Lord, they are satisfied with the goodness and fatness thereof, and are made to drink of the river of divine pleasure; and being in the courts of the Lord, where a feast of fat things is provided for them, they eat and feed, and so thrive and flourish; the allusion is to fat and flourishing palm trees (g).
(g) "Praeferat Herodis palmetis pinguibus----". Horat. Ep. l. 2. Ep. 2. v. 148.

Old age - Their last days shall be their best days, wherein they shall grow in grace, and increase in blessedness.