by the time they are thirty：等他们到了三十岁时

这是由by the time 引导的一个时间状语从句。句子不完整（缺少主句）。

by the time 可以直接用作连词，引导时间从句。由名词time 构成的连词归纳和举例：

The truck will have arrived by the time you have all the things packed up. 

到你把一切东西都捆好时，卡车也就来了。

英语中，the time, the first time, next time, any time, every time，each time, the last time, from the time, by the time 等，都可以作连词用，引导时间状语从句。

The first time I climbed onto the wall, I felt nervous.

我第一次爬到墙上去时，感到胆怯。　　　

Every time I catch a cold, I have pain in my back. 

我每次感冒背就痛。

Any time you come to London do look me up.  

你无论什么时候到伦敦来，一定要来看我。

I recognized you the minute I saw you. 

我一见你就认出了你。　　　　　

Next time you come, you will see the scientist. 

你下次来时就会见到那位科学家。　　　　

You may come back any time you want to. 

你想回来随时可回来。　　　　

Each time the tourist traveled by boat he got seasick. 

这位旅客每次坐船行时都晕船。　　　　

The last time she saw James, he was lying in bed. 

她最后一次看詹姆斯时，他躺在床上。