Children don't complete their set of 20 baby teeth until they are two to three years old.

孩子到两三岁时才长齐20颗乳牙。

《柯林斯英汉双解大词典》

When I was eight years old, my sister was three years old.

我八岁的时候，我妹妹三岁。

youdao

He could find the way home though he was only three years old.

他虽然只有三岁，却能找到回家的路。

youdao

He was fifty-three years old. J.Robert Oppenheimer and Enrico Fermi were two of the greatest scientists of the century.

VOA: special.2009.06.14

I'm three years old. Maybe I don't know what "pulls up" means.

我三岁，可能我不知道“停下来“是什么意思。

耶鲁公开课 - 文学理论导论课程节选

And then you remember that you moved when you were about three years old.

这时你记起大概三岁的时候，你们搬过一次家。

I used to 课堂 - SpeakingMax英语口语达人

And consider that the merger between Sanofi-Synthelabo and Aventis is not yet three years old.

FORBES: Why Bristol And Sanofi Shouldn't Merge

In the wild, tragopans do not normally breed until they are three years old.

BBC: Tragopan pheasants

The current rules stipulate one member of staff for every eight children over three years old.

BBC: Cut nursery staff to child ratio, urges minister