Yeats exhorts his beloved: when you are old and falling asleep by your fire, take down this book, and dream of how you used to be as you read it.
Dream of how many people loved you when you were younger. Only one man loved you as you grew older.
Murmur to yourself sadly about how Love paced on the mountains and hid his face in stars as you grew old.
Like so many of the poems in this collection, "When You Are Old" was written for Maud Gonne. It is based on Ronsard's "Quand Vous Serez Bien Vieille," Sonnets Pour Helene (1578), which maintains the Maud Gonne/Helen of Troy parallel that Yeats so often draws. The idea of love in age is an ancient one, meant to express the fact that love inheres not merely in youth, but in something deeper and more lasting.
Yeats capitalizes "Love," thus personifying the concept, which is is a nod to the poem's 16th century roots. Although monotheism had taken over Europe, Greek and Roman gods were very much a part of 16th century consciousness. Yeats's "Love" is a modernization of the ancient figure, Eros.