《当你老了》是爱尔兰诗人威廉·巴特勒·叶芝的一首著名诗歌，原文如下：
```
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.
```
这首诗是叶芝献给他的友人茅德·冈的，表达了他对茅德·冈的热烈而真挚的爱情。诗中，“当你老了，头发花白，睡意沉沉”描绘了一个人年老色衰的景象，而“唯有一人爱你灵魂的至诚，爱你渐衰的脸上愁苦的风霜”则表达了一种超越时间和外表的深情。整首诗歌通过对比青春和衰老，展现了爱情的永恒和忠诚。