Sonnet 73 is a work of exhortation, the speaker talks about how his life becomes old. He exhorted his lover to seize the moment and love each other heartily. Sonnet 73 uses autumns, twilight, and a dying fire as extended metaphors for growing older. The poem makes it clear that aging and death are inevitable, but it also affirms that the person the speaker is addressing still loves the speaker anyhow.
The first quatrain compares the speaker's life to the end of the summer—Autumn an image of bare, lifeless boughs after a vital summer. The first quatrain means: “When you look at me you must see that time of year when yellow leaves, or no leaves, or just a few leaves, hang on tree branches that shiver in the cold.” The poem suggests that human life is bound by the laws of nature. The first description, of late autumn, demonstrates time’s power to destroy that which was once whole and beautiful. Trees are described through their autumnal “yellow leaves” that falls to the ground. They all suggest decay, the poem first compares the speaker to a tree in the late autumn, which on one level reveals the speaker's advancing age. Concurrently, however—though this tree is clearly showing signs of decay with its yellow, falling leaves and shivering 'against the cold'—this tree is not yet dormant for the winter.
The second metaphor then compares the speaker’s current time of life to “twilight,” which is the time when the day’s last light is still present in the sky, but the dark night is imminent. This suggests that the speaker's 'light'—his vitality, attractiveness, wit, or any number of other qualities—has peaked, has already come and gone; everything is only going to get darker.
Last, the speaker compares his ending life to a dying fire. “In me, you can see the glow of a dying fire that rests on its own ashes like a deathbed, since the fire will eventually burn out upon the remains of the wood that once fueled it.” The connection between old age and death is made explicit: the fire is on its 'death-bed' and 'must expire,' suggesting that the speaker, too, is edging ever closure to his own expiration date. I think this one can be truly considered an accurate metaphor for growing old, because The flame is fading and dying out, and he grows dim and nears extinction as a dying fire.
The poem makes it clear that aging and death are inevitable, and the poem focuses on the uncompromising movement of time toward death, this natural process is not presented as a negative thing.