The Romantic tradition in poetry tended to associate love poetry with youth, both in the sense of the protagonists of love poems being young and of the readership also being relatively young. Several of Yeats' poems, though, explore love as something remembered, reflected upon, or fully realized in old age and art.
Perhaps the best known poem in which Yeats addresses the paradox of love and aging is "When You Are Old", in which the narrator explains that others loved the woman of the poem for her external beauty, but one man (the narrator) loved her soul. He imagines that reading the poem in her old age, she will come to understand this, but it will be too late for the opportunity for love will have fled. Thus while the young enjoy the external reality of love, the mature wisdom about love is expressed in art, which is what they reflect upon in their old age, when they no longer are actually engaged in the activities the art describes.
This paradoxical inversion of the relationship among youth, love, art, the soul, and old age can also be seen in "Sailing to Byzantium" which begins with the line "That is no country for old men." The young are portrayed as caught in "sensual music". A deeper and more spiritual form of art though appears in the soul of the aged, which rather than trying to recapture lost youth, sees as the model for eternal truths the ageless sages of Byzantine art and the mechanical bird of the final stanza, which does not participate in human mortality.