man of perennial youth; man who never loses his youthful vigor
Describes a man who remains youthful in spirit and energy regardless of age. Often used in a positive, admiring sense.
He is 80 years old, but he is as energetic as a man of perennial youth.
Compound of 万年 (ten thousand years, eternity) and 青年 (young man). The exact origin is uncertain, but it is a straightforward combination expressing the idea of eternal youth.