。
She isn't as young as she looks.
Literal
She [topic-は] appearance to-the-extent-of young [contrast-は] not.
An AはBほど~ない comparison ('A is not as ~ as B'), with one extra wrinkle: the は inside 若くはない. Tucking は between the い-adjective stem 若く and the negative ない adds contrastive emphasis — it isolates 'young' as the specific quality being denied, leaving the door open for an unstated contrast ('she's not young, but she's something else'). It's a subtle particle-stacking move that becomes second nature with exposure.