Thou hast shortened the days of his youth. Some would explain this sentence as meaning, that God had weakened the king, so that he faded or withered away at his very entrance upon the flower of youth, and was exhausted with old age before reaching the period of manhood. This exposition may be regarded as not improbable; but still it is to be observed, in order to our having a clearer understanding of the mind of the prophet, that he does not speak exclusively of any one individual, but compares the state of the kingdom to the life of man. His complaint then amounts to this, That God caused the kingdom to wax old, and finally to decay, before it reached a state of complete maturity; its fate resembling that of a young man, who, while yet increasing in strength and vigor, is carried away by a violent death before his time. This similitude is highly appropriate; for the kingdom, if we compare the state of it at that period with the Divine promise, had scarce yet fully unfolded its blossom, when, amidst its first advances, suddenly smitten with a grievous decay, its freshness and beauty were defaced, while at length it vanished away. Moreover, what we have previously stated must be borne in mind, that when the prophet complains that the issue does not correspond with the promise, or is not such as the promise led the chosen people to expect, he does not, on that account, charge God with falsehood, but brings forward this apparent discrepancy for another purpose -- to encourage himself, from the consideration of the Divine promises, to come to the throne of grace with the greater confidence and boldness; and, while he urged this difficulty before God, he was fully persuaded that it was impossible for Him not to show himself faithful to his word. As the majority of men drink up their sorrow and keep it to themselves, because they despair of deriving any benefit from prayer so true believers, the more frankly and familiarly they appeal to God in reference to his promises, the more valiantly do they wrestle against their distrust, and encourage themselves in the hope of a favorable issue.

The days of his youth hast thou shortened - This does not mean that he had shortened his life, but that he had abbreviated the period of his vigor, his hope, and his prosperity; instead of lengthening out these, and prolonging them into advancing years, he had by calamities, disappointments, reverses, and troubles, as it were, abridged them. No such youthful vigor, no such youthful hope now remained. The feelings of age - the cutting off from the world - had come suddenly upon him, even before he had reached the season when this might be expected to occur. Though at a time of life and in circumstances when he might have hoped for a longer continuance of that youthful vigor, he had suddenly been brought into the sad condition of an old man.
Thou hast covered him with shame - Hast clothed him with shame or disgrace. Everything in his circumstances and in his appearance indicates shame and disgrace, and the divine displeasure.

The days of his youth hast thou shortened - Our kings have not reigned half their days, nor lived out half their lives. The four last kings of Judea reigned but a short time, and either died by the sword or in captivity.
Jehoahaz reigned only three months, and was led captive to Egypt, where he died. Jehoiakim reigned only eleven years, and was tributary to the Chaldeans, who pat him to death, and cast his body into the common sewer. Jehoiachin reigned three months and ten days, and was led captive to Babylon, where he continued in prison to the time of Evilmerodach, who, though he loosed him from prison, never invested him with any power. Zedekiah, the last of all, had reigned only eleven years when he was taken, his eyes put out, was loaded with chains, and thus carried to Babylon. Most of these kings died a violent and premature death. Thus the days of their youth - of their power, dignity, and iife, were shortened, and they themselves covered with shame. Selah; so it most incontestably is.

The days of his (f) youth hast thou shortened: thou hast covered him with shame. Selah.
(f) He shows that the kingdom fell before it came to perfection or was ripe.

The days of his youth hast thou shortened,.... His days of joy and pleasure; such as days of youth are, in opposition to the days of old age, which are evil, Ecclesiastes 11:9, these were shortened when his sorrows and sufferings came on, and God hid his face from him; and indeed he was a man of sorrows, and acquainted with grief all his days: the Vulgate Latin version renders it, "the days of his time"; and the Arabic version "the days of his years"; for he did not live out half the time of man's age, which is threescore years and ten, Psalm 90:10, he dying at the age of thirty three or four; but, notwithstanding this, he lives again, and lives for evermore; he has length of days for ever and ever, Psalm 21:4, though his days were in some sense shortened, yet in another sense they are and will be prolonged, even his own, and those of his spiritual seed, according to the promise of God, Isaiah 53:10,
thou hast covered him with shame; see Psalm 69:7, when his face was covered with shame and spitting, from which he hid it not, Isaiah 1:6, but now he is crowned with glory and honour; wherefore all these complaints, though true, are no objections to what is before said and swore to.
Selah. See Gill on Psalm 3:2.

days of his youth--or, "youthful vigor," that is, of the royal line, or promised perpetual kingdom, under the figure of a man.

Youth - The youthful and flourishing estate of David's kingdom was very short, and reached not beyond his next successor.