expression, godan verb (-ru)
turn 50; reach 50 years of age
A classical-sounding expression derived from Confucian thought, where 天命 (Heaven's mandate) was understood to be known at age 50. Used as a literary or formal way to say one has turned 50.
彼は今年で天命を知る年齢になった。
He has reached the age of 50 this year.
天命を知るという言葉
The phrase 'knowing Heaven's mandate' originates from the Analects of Confucius.