If we were to remove the final two lines, we could say that this poem was just about old age and dying.
As per his poem/sonnet youth is definitely bright and filled with fire, but sooner the fire gets vanish somewhere or dies out as per the growing age.
It is so because Shakespeare has described the relation of old age to death. He tried to define the emotion, feelings of the time when a person knows his/her time has come to leave the world.
By describing so he even used the metaphors like autumn and twilight for the old age. As the leaves gets faded and shed from the trees in autumn.
To learn more about autumn here
brainly.com/question/1098749
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