He was barely 20 years old and already running his own company.
他只有20岁，却已经营起自己的公司了。
I was twenty years old and ready for anything.
我当时二十岁，什么都愿意去做。
These rocks have been dated radiometrically at two billion years old.
经辐射能探测，这些岩石形成于20亿年前。
Young children are beginning their formal education sometimes as early as four years old.
幼儿有时早在四岁时就开始接受正规教育。
In those days most people left school when they were only fifteen years old.
那时候，大多数人上学只上到十五岁。