《当你老了》是威廉·巴特勒·叶芝的一首著名诗作，以下是其原文：
```
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.
```
这首诗以温柔而深情的笔触，描绘了一个人年老色衰时的情景，同时回忆往昔的美好时光和爱情的忠贞不渝。