。
She isn't as young as she looks.
Literal
She [topic-は] outward-appearance [as-much-as-ほど] young [contrast-は] is-not.
X ほど Y ない is the standard pattern for 'not as Y as X' — X is the benchmark, Y is the quality, and the negative says the subject doesn't reach it. So 外見ほど若くはない = 'isn't as young as her outward appearance.' Notice the は after 若く: instead of plain 若くない, the speaker inserts は to mark a contrast — 'young, in contrast to other things one might say about her, she is not.' This contrastive は in negative i-adjective predicates is everywhere in careful or slightly emphatic Japanese, and learners often miss it on a first read.