。
She's not young, is she?
Literal
She [topic-は] young [contrast-は] is-not [right?-ね].
若くはありません uses contrastive は to narrow the negation — she's specifically not young (implying other positive qualities may exist). The polite negative ありません and the sentence-final ね (seeking agreement/confirmation) together create a gentle, confirmatory tone: the speaker expects the listener to agree with this observation.