“When You Are Old” is a bittersweet poem that reveals the complexities of love. The poem is generally taken to be addressed to Maud Gonne, an Irish actress with whom Yeats was infatuated throughout his life (which is why we're using male and female pronouns in this guide). That said, the poem can also be interpreted more broadly, without specifying the names or genders of either the speaker or the addressee. In any case, the poem argues in favor of a kind of love based not on physical appearances—which fade over time—but on the deeper beauty of the soul.
In the first stanza, the speaker asks the addressee to think ahead to a time when she will be old, tired, and grey. Then, says the speaker, the addressee will look back nostalgically on her life to date, thinking of her youthful looks and vigor as though they were a dream. Those who love the addressee now—that is, at the time of the poem's writing, when this woman is ostensibly still young—are portrayed as superficial and insincere. The speaker implies that the so-called love of these men for the addressee will fade, just as the basis for that love—the addressee’s beauty and youth—will fade too.
The speaker contrasts his own love for the addressee with the inferior love described above. The speaker’s love, the poem argues, will stand the test of time because it is based on the addressee’s “pilgrim soul” and the “sorrows” of her “changing face.” That is, the speaker perceives an inner restlessness of this woman's soul and implies that this will express itself in her “changing face” as she grows old. The speaker, then, claims to experience love that goes beyond the surface—the addressee's face may change over time, but the "soul" that the speaker loves will not.
Furthermore, the pilgrim-like quality of the addressee’s “soul” might be the very reason why she seems to have denied the speaker’s love. It sounds like the addressee refuses to settle down—meaning that the speaker is expressing love not just in spite of being rejected, but in part because of it too.
With the above in mind, though, the speaker isn’t exactly painting a rosy picture of the addressee’s future. In essence, the speaker is predicting a lonely scene, one in which this woman has only a fire and a book for company. Indeed, the speaker predicts that it will be through reading “this book”—the one in which the poem appears—that the addressee will be reminded of her youth and, ultimately, her failure to embrace love when given the chance. The speaker is suggesting that the poem itself will stand as a testament to the speaker's true form of love, when the shallow love of others is nothing but a distant memory.
The poem expresses a complicated sentiment, then, attesting to the power of love as well as its limits. Indeed, there is a hint of bitterness in the way the speaker predicts that the poem itself be a reminder of how love “fled” from the addressee. But whatever the complexities, there is no doubting the speaker’s strength of feeling—and through the poem, the reader is reminded that true love of the kind described is rarely simple, easy, or certain.