问题补充：
the boy could skate when he was four years old同义句
答案：
The boy was able to skate when he was four years olds.
The boy could skate/was able to skate at the age of four.
======以下答案可供参考======
供参考答案1:
The boy started playing the piano at the age of four.
翻译：这个男孩四岁时开始弹钢琴。
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