He was barely 20 years old and already running his own company.

　　他只有20歲，卻已經營起自己的公司了。

　　I was twenty years old and ready for anything.

　　我當時二十歲，什么都愿意去做。

　　These rocks have been dated radiometrically at two billion years old.

　　經輻射能探測，這些巖石形成于20億年前。

　　Young children are beginning their formal education sometimes as early as four years old.

　　幼兒有時早在四歲時就開始接受正規教育。

　　In those days most people left school when they were only fifteen years old.

　　那時候，大多數人上學只上到十五歲。