also: まんねんしんぞ
noun
woman of perennial youth
A somewhat dated or literary term for a woman who appears perpetually young. Often used in a complimentary or slightly romanticized way.
彼女はまるで万年新造のように若々しい。
She looks as youthful as a woman of perennial youth.
若作り refers to dressing or making oneself up to look younger, often with a slightly negative nuance of trying too hard, while 万年新造 is a more poetic term for someone who naturally retains youth.
Compound of 万年 (ten thousand years, eternity) and 新造 (newly made, or a young woman). The exact historical derivation is uncertain, but it figuratively describes a woman who seems forever young.