The poem begins with a restatement of its title, setting up what follows as direct communication between the speaker and an unnamed addressee. This addressee is usually taken to be Irish Nationalist and actress Maud Gonne, but the poem works equally well with a more general interpretation. Regardless of the precise identities of the speaker and addressee, the poem feels intimate from the beginning, with the reader as an outside observer of a seemingly close relationship.
"When You Are Old" contrasts two moments in time. The first line asks the addressee—and by extension the reader—to think beyond the present moment and imagine the future. This is a time when the addressee will be "old," "grey," and sleepy. This future scene contrasts with the time of the poem's writing, which the reader learns more about from line 3 onwards.
The first line subtly evokes the weariness of old age. The hypnotic /l/ consonance in "old," "full," and "sleep" has a lulling effect, conjuring an image of the addressee drifting in and out of sleep by the fire. The repeated use of "and"—which is developed into more extensive polysyndeton later in the poem—has a similar effect. The "and" between "old" and "grey" is grammatically unnecessary, but its presence makes the line take just a little bit longer, slowing the poem down to a pace befitting a vision of sleepy old age.
In line 2, the poem starts to reveal its speaker's agenda. The speaker offers the addressee an instruction for this far-off future moment: "take down this book." The preceding caesura and the gentle /d/ consonance keep up the sleepy atmosphere. It's not immediately obvious which book the speaker is referring to, but as the mode of communication here—the poem itself—is a form of literature, it could well mean the book in which this poem is collected. This interpretation would support the idea that this poem is, on at least one level, about the complicated relationship between Yeats himself and his muse, Maud Gonne. Regardless, the speaker feels that this specific book will, in the future, tell the addressee something about her life, particularly with regard to the contrast between the poem's present and the future it projects.