Confucius said at fifty one knows heaven's mandate — only now at this age am I slowly feeling it.
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history
From 《论语·为政》, Confucius's self-biography: 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。(At fifteen I set my heart on learning; at thirty I stood firm; at forty I had no doubts; at fifty I knew the mandate of heaven; at sixty my ear was attuned; at seventy I could follow my heart without overstepping.) This is why 50 is called 知天命之年 in Chinese.
usage
In modern Chinese, 五十 is used as a byword for 'age fifty' thanks to this passage — e.g. greeting cards, obituaries, and life-milestone writing. Pairs with 三十而立 (thirty, standing firm) and 四十不惑 (forty, no doubts).
