The prime of life: middle-aged years when a person is typically at their physical and mental prime (often around 30s–50s).
彼はまだ壮年で体力もあり、仕事に張りがある。
He is still in his prime and has energy, showing vigor in his work.
壮年期に健康管理を始めることが大切だ。
It's important to start managing your health during the prime years.