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prime of one's working life
働き盛りの彼が部長になった。
He, in the prime of his working life, was promoted to department head.
働き盛りで過労死するケースが増えている。
Cases of people dying from overwork in the prime of their careers are increasing.
彼女は会社の海外展開と同時に働き盛りを迎えた。
She reached the prime of her working life just as the company began its overseas expansion.