。
How young she looks!
Literal
She [topic-は] what young [explanatory-の] [I wonder/exclamation-でしょう].
若い ('young') is an i-adjective predicate, with の nominalising before the polite exclamatory でしょう. In Japanese, commenting on someone's youthful appearance — 若く見える ('looks young'), お若いですね ('how young you are!') — is a near-universal compliment, usually well-received. This contrasts with some Western cultures where age-related comments can feel intrusive. The cultural emphasis on aging gracefully and the prized quality of 若々しさ ('youthfulness') makes such remarks routine social currency.