题目内容：

He started swimming when he was four years old .(同义句) He started swimming（ ）（ ）

优质解答

He started swimming（at age）（ four）.
或
He started swimming（at the age of four ）.
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追问：

一个括号中一个词

追答：

要是这样，只能是：He started swimming（at ）（ four）. 不过常见的是上面的两种。