“a dog's age”别理解成“一条狗的年龄”，其正确的含义是好久；很长时间。

【例句】
It's been a dog's age since I worked one of these machines, but I'll give it a shot!
我已经好久没有用过这些机器了，但我还是要试一试！

I haven't seen you in a dog's age. How's life treating you?
我很长时间没有看到你了。你最近过得怎么样？