、。
Mother looks young for her age.
Literal
Mother [topic-は], age [gen-の] proportion [for-には] young appears (polite).
~のわりには is the set pattern for 'considering X, (surprisingly) Y' — a comparative that measures Y against the expectation set by X. Here 'for her age' = 'considering her age.' The ~く + 見える construction is 'appears [adjective],' with 見える as 'be visible/appear.' A compact, genuinely useful phrase.