prime of one's working life; peak working years
Refers to the period in life when a person is most active and productive in their career, typically middle age. Often used in contexts discussing work, health, or social contributions.
He fell ill in the prime of his working life.
The generation in their peak working years supports society.
He's in his prime working years, in his 40s.
My father collapsed from illness right in the prime of his working years.