Though the root thereof wax old - Though life becomes almost extinct. The idea is, though the root of the tree be very old, yet it does not become wholly lifeless. It is not like an old man, when life goes out altogether. In the very aged root there will be vitality still; but not so in man.
Though the stock thereof - The stump - literally that which is cut off - גזעוּ geza‛ô. The meaning is, that when the trunk of the tree is cut down and dies altogether, life remains in the root; but when man fails, life is wholly extinct.

Though the root thereof wax old in the earth,.... Lies long there, and is become dry, and seems to be consumed, on which account there may be the less hope of its flourishing:
and the stock thereof die in the ground; which may make it still more improbable; for this is not to be understood with some interpreters (y) of the stock or trunk of the tree cut down, and lying along on the earth, and in the dust of it; though it may be observed, that even such a stock or trunk, separated from the root, and as it lies along, will sprout again, as particularly in elms: but it may rather mean, since it is said to be "in the ground", that part of the stock or stump left in the ground, from whence the roots part and spread in the earth; and even though this dies, or at least so seems, yet there being still life and vigour in the roots, they send forth suckers.
(y) So Piscator and Cocceius.

Die - To outward appearance.