in the prime of life

in the prime of life (at the peak of one's creative abilities; during the most productive years. ) — в расцвете (лет, сил)
a man in the prime of life

Example: Poor Jack lost his job due to restructuring when he was in the prime of his life.

Also
in one’s prime

Example: She's now fifty and still in her prime.

[at the peak of]
[в расцвете]

Opp.
past one's prime (He is now past his prime.)
[over the hill]