prime of life; the prime years
Refers to the period of life when a person is at their physical and mental peak, roughly from the late 20s to early 50s. Often used in formal or written contexts.
He is in the prime of life and tackles his work with the greatest energy.
Health management during one's prime years greatly influences the rest of one's life.
He has finally reached the prime of his life and is showing steady strength at work.
A man in his prime swung the hammer down with great force.