Charles H. Spurgeon on Psalms 90:5

They are like grass. In this last similitude, the prophet compares men to grass, that as grass hath a time of growing and a time of withering, even so has man. In the morning it flourisheth, and groweth up. In which words Moses compares the former part of man's life, which is the space of thirty-three years, to the time of growing of grass, and that is accounted the time of the perfection of man's strength and age; at which age, according to the course of nature, man flourisheth as grass doth; that is the time of a man's prime and flourishing estate. But in the evening; that is, when the grass is ripe, and ready to be cut down, it withereth. Even so man, being once at his strength, and ripest age, doth not stand at a stay, nor continueth long so; but presently begins to decay, and to wither away, till old age comes, and he is cut down by the scythe of death. Now, in that Moses useth so many similitudes, and all to show how frail this life of man is, we are taught, that the frailty, vanity, and shortness of man's life is such, that examples will scarcely shew it. Death comes as a flood, violently and suddenly; we are as a sleep; we are as grass; our life is like a dream; we spend our days as a tale that is told, Ps 90:9. All these similitudes Moses hath in this Psalm, as if he wanted words and examples, how to express the vanity, frailty, and shortness thereof. —Samuel Smith.