The image of twilight "after sunset fadeth in the west" is the second of three vivid metaphors for old age, each of which occupies one quatrain of the sonnet. The first of these uses trees in Autumn when the leaves have turned yellow or fallen from boughs where the birds no longer sing. The second is the twilight, "quickly overtaken by night, Death’s second self."

The image in the next quatrain is one of a dying fire “That on the ashes of his youth doth lie.” The fire is still smoldering and glowing with heat but its light is dying and, like the twilight, it is perilously close to being extinguished by darkness. The poet says that the fire which nourishes youth consumes old age, a striking image, suggesting that everyone has a finite amount of fuel and when that is gone, we are nothing but feebly glowing ashes, soon to turn cold.

Although Shakespeare is writing to a younger man (with whose youth he often contrasts his own age in other sonnets), he is scarcely decrepit himself. He can scarcely be much older than thirty-four (which would date the sonnets to 1598, one of the latest realistic possibilities) and may even be as young as twenty-eight. The depiction in this sonnet of an old man close to death, therefore, is a remarkable feat of imagination.