请问一下at the age of 与 by the age of 的区别.

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请问一下at the age of 与 by the age of 的区别.
还有最好给一下例句.
__the age of twenty,he had written two books.

at the age of 在某一岁数；在……岁时
by the age of 在某一岁数之前；到……岁时
Incredibly he shot a film at the age of 17.
不可置信的是他17岁就拍了部电影.
He must learn to be independent by the age of 18.
他18岁前必须学会独立.
你补充的问题答案用by

请问一下at the age of 与 by the age of 的区别. "by the age of","at the age of"的区别? in the age of by the age of at the age of的区别 by the age of 和at the age of的区别一些关于介词的用法at the age of by the age of有什么区别 at the age of与at a age of的区别 by the age of at the age of at the same age 和 of the same age 的区别? 英语at the age of? 翻译 at the age of at the age of five