The boy started playing the piano when he was four years old

題目：

The boy started playing the piano when he was four years old.(同義句）
The boy started playing the piano ___ ___ ___ ___ ___ .

解答：

正確答案：填at the age of four
即：The boy started playing the piano at the age of four.
翻譯：這個男孩四歲時開始彈鋼琴.
有不明白的請繼續追問,