W. B. Yeats, (13 June 1865 – 28 January 1939), was an Irish poet and writer, and one of the leading literary figures of the last century. He is a key figure behind the Irish Literary Revival. In December 1923, Yeats was awarded the Nobel Prize in Literature, “for his always inspired poetry, which in a highly artistic form gives expression to the spirit of a whole nation”. From my research, I believe When You Are Old was published in 1893, when Yeats was not very old. It was written as a love poem to an Irish revolutionary who stole his heart, Maud Gonne. She refused to marry him despite several proposals. See the poem below:

When you are old and grey and full of sleep,
And nodding by the fire, take down this book,
And slowly read, and dream of the soft look
Your eyes had once, and of their shadows deep;
How many loved your moments of glad grace,
And loved your beauty with love false or true,
But one man loved the pilgrim soul in you,
And loved the sorrows of your changing face;
And bending down beside the glowing bars,
Murmur, a little sadly, how Love fled
And paced upon the mountains overhead
And hid his face amid a crowd of stars.
The phrase ‘full of sleep’ is quite horrifying to me. It implies a stage in one’s old age when they are so tired and close to death that they are almost sleep walking into death. Now, death is not necessarily something to be afraid of, especially not for a practicing Catholic. But this stage of life where one is exhausted is quite worrisome for someone with a modicum of vitality, at the time of writing. The first two lines invite Maude to read this poem while she is within a few weeks of death and comfortable by the fire, possibly dozing off.
The next two lines invite Maude to remember her youth, indeed dream of it. The dark shadows signify pessimism and depression, perhaps.
The second stanza in my view casts aspersions at Maude for dumping Yeats. It implies that the man she went on to marry was a false lover. Perhaps, those who loved her falsely enjoyed her beauty over the substance of her character. This is confirmed in the second half of this stanza when Yeats points out, helpfully, that he loved her wandering and/or searching soul
The last stanza reminds me of the beginning of Revelation 12, where the writer (ostensibly St John), describes the Virgin Mary in Heaven.
A great sign appeared in heaven: a woman clothed with the sun, with the moon under her feet and a crown of twelve stars on her head.
Yeats is berating his ex love interest in the spiciest form yet, goading her for wasting an entire life because she has not spent it with him, even implying she is going to hell as he, Mr Right, ends up in heaven. Overall, a lovely poem! There is a lot of sass, a lot of projection and a lot of power here.