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。

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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。

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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

When Mike was a kid of about three years old, he was playing at his parents' garage.

迈克大约三岁的时候，一天他在父母的车库里玩耍。

斯坦福公开课 - 7个颠覆你思想的演讲课程节选

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