woman of perennial youth
A playful or slightly teasing term for a woman who always looks young, or who acts like a young girl regardless of age. Often used in a lighthearted, colloquial context. Compare with 万年新造.
See also: 万年新造
She's so youthful all the time, like a perennial girl.
Compound of 万年 (man'nen, 'ten thousand years, eternity') and 娘 (musume, 'daughter, young girl'). The exact origin is uncertain, but it likely emerged as a colloquial expression.