I am not one to usually borrow or use a commentary, but Matthew Henry is well respected, so I chose to use this:
II. There shall be new life, v. 20. Untimely deaths by the sword or sickness shall be no more known as they have been, and by this means there shall be no more the voice of crying, v. 19. When there shall be no more death there shall be no more sorrow, Rev. xxi. 4. As death has reigned by sin, so life shall reign by righteousness, Rom. v. 14, 21. 1. Believers through Christ shall be satisfied with life, though it be ever so short on earth. If an infant end its days quickly, yet it shall not be reckoned to die untimely; for the shorter its life is the longer will its rest be. Though death reign over those that have not sinned after the similitude of Adam's transgression, yet they, dying in the arms of Christ, the second Adam, and belonging to his kingdom, are not to be called infants of days, but even the child shall be reckoned to die a hundred years old, for he shall rise again at full age, shall rise to eternal life. Some understand it of children who in their childhood are so eminent for wisdom and grace, and by death nipped in the blossom, that they may be said to die a hundred years old. And, as for old men, it is promised that they shall fill their days with the fruits of righteousness, which they shall still bring forth in old age, to show that the Lord is upright, and then it is a good old age. An old man who is wise, and good, and useful, may truly be said to have filled his days. Old men who have their hearts upon the world have never filled their days, never have enough of this world, but would still continue longer in it. But that man dies old, and satur dierum--full of days, who, with Simeon, having seen God's salvation, desires now to depart in peace. 2. Unbelievers shall be unsatisfied and unhappy in life, though it be ever so long. The sinner, though he live to a hundred years old, shall be accursed. His living so long shall be no token to him of the divine favour and blessing, nor shall it be any shelter to him from the divine wrath and curse. The sentence he lies under will certainly be executed, and his long life is but a long reprieve; nay, it is itself a curse to him, for the longer he lives the more wrath he treasures up against the day of wrath and the more sins he will have to answer for. So that the matter is not great whether our lives on earth be long or short, but whether we live the lives of saints or the lives of sinners.
MHCC 2002
That should clarify the meaning better.