This brilliant poem has clearly a lot more to say than simply the narrow focus that some critics give it concerning the themes of aging. The appeal of the speaker is to the transformative power of art and in particular the way in which art is able to express what is indescribable and to leave the self behind as it boldly strikes out towards new visions of humanity and the world.
The poem at its heart is about an old man's desire to leave the "sensual music" of Ireland which he finds so meaningless and metaphorically "sail to Byzantium," which acts as a symbol of the transformative power of art. The end goal of the speaker is complete transformation in himself as he seeks to leave his earthly body behind and find new life in a very different form. Consider what the last stanza tells us:
Once out of nature I shall never take
My bodily form from any natural thing,
But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy Emperor awake;
Or set upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.
The speaker, having shed his earthly body behind, through the power of art wishes to evolve his consciousness into a different form and forge some kind of supernatural union with the art of this medieval empire, allowing this new art that he becomes to give him the powers of a prophet. This transformation of this frail human bound to its earthly body to a golden bird who is able to achieve a kind of immortality is much to be favoured by the speaker, who sees art as being much more superior than life itself.