Refers to the period when a person is at their physical and mental peak, typically middle age. Often used in literary or formal contexts.
He achieved great success in the prime of his life.
stage of maturity
Geological term for a mature stage in landscape development, such as a river valley or landform. Technical use.
This landform shows characteristics of the stage of maturity.
老年期 refers to old age, the stage after 壮年期.
Compound of 壮年 (sōnen, 'prime of life') + 期 (ki, 'period'). The geological sense is a calque from English 'stage of maturity'.