at the age of/in one's + 基数词复数/by the age of
（1）The hero of the story is an artist in his ____.
A. thirtieth B. thirty C. thirty's D. thirties(全国高考,91-36)
(2) He looked quite healthy though he was ____.
A. in seventy B. in his seventy
C. at seventies D. at the age of seventy(上海高考,93-20)
《2000 英语高考题型解析》(华东理工大学出版社, p.76) 对试题(1)作了如下解答:
“in one's + 基数词复数表示‘年龄’、‘岁’”。照此解释，它不是跟 at the age of 同义了吗？
at the age of 作“在……岁时”解,表示的是准确的岁数,而 in one's + 基数词复数作“在几十多岁时”解,表示的是非精确的岁数。请对比:
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I was born and brought up in the small town of Pineville, which I left at the age of sixteen. （全国高考）我生长在帕尼维尔小镇，我16岁时离开了它。
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At the age of eight, he joined a group of child dancers. 他8岁时参加了儿童舞蹈团。
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Edgar Snow first came to China in 1928, at the age of twenty-two. 1928 年, 埃德加·斯诺 22 岁首次来到中国。
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Marx was already in his fifties. 马克思已经五十多岁了。
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He is still in his twenties. 他才二十多岁。
by the age of 则是指“在……岁之前”“到……岁时”：
By the age of 21 he had already begun to notice that it was difficult to move around without falling over. 到 21 岁时,他就已经察觉到他一走动就难免摔倒。