他在五十几岁的时候出名了.He became famous ____ his ____.

He became famous in his fifties.

in fifties
in one's +几十的复数，是某人的几十几岁

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以上两个都错了,正确的是：He became famous （in ) his( fifties )
在某人几十岁：in+ one's + 整十数词的复数形式.
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He became famous __in__ his _fifties ___.
表示“在某人十几岁/二十几岁/三十几岁……九十几岁”时，用介词in,即构成短语“in one’s teens/twenties/thirties…nineties”。如：
When Marx was already in his fifties,he found it important to study the situation in Russia.当马克思五十几岁时，他发现研究俄国形势很重要。