He turned forty this year - truly 'free from doubt at forty', increasingly decisive in his actions.
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history
From 《论语·为政》 (Analects). Confucius's famous autobiography: 吾十有五而志于学，三十而立，四十而不惑，五十而知天命，六十而耳顺，七十而从心所欲不逾矩。 The six ages became the standard Chinese framework for life stages; 不惑 is now a common euphemism for 'forty years old.'
usage
Almost never said alone - quote as part of the set 三十而立、四十而不惑、五十而知天命. In modern Chinese, 不惑之年 is a polite phrase for someone in their forties.
