当你开始回忆的时候，证明你已经老了。 原句是什么？

这是叶芝的一首诗《当你老了》，英文原版

When you are old

--- William Butler Yeats

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.

中文翻译有很多版本，列举一种：当你老了

袁可嘉译

当你老了，头白了，睡意昏沉，

炉火旁打盹，请取下这部诗歌，

慢慢读，回想你过去眼神的柔和，

回想它们昔日浓重的阴影；

多少人爱你青春欢畅的时辰，

爱慕你的美丽，假意或真心，

只有一个人爱你那朝圣者的灵魂，

爱你衰老了的脸上痛苦的皱纹；

垂下头来，在红光闪耀的炉子旁，

凄然地轻轻诉说那爱情的消逝，

在头顶的山上它缓缓踱着步子，

在一群星星中间隐藏着脸庞。