noun
prime of life; heyday; flourishing years
Refers to the period when a person is at their peak physically, mentally, or in their career. Often used in formal or literary contexts.
彼は盛年にして、すでに大きな業績を残した。
He achieved great things while still in his prime.
盛年を過ぎても、彼の情熱は衰えなかった。
Even past his prime, his passion never waned.
Composed of 盛 (flourishing, prosperous) and 年 (year, age). The exact historical derivation is uncertain, but the combination naturally conveys the idea of one's flourishing years.