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She is surprisingly energetic for her age.
Literal
She [topic-は] age's [for-割には] [surprise-driving extent-驚くほど] energetic is.
Two adjacent grammar points. ~の割には ('for, considering ~') sets up the standard against which she's being measured. 驚くほど ('to a surprising extent') uses ほど ('extent') to modify 元気 — literally 'to the degree that one is surprised.' This ほど construction is productive: 信じられないほど ('unbelievably'), 死ぬほど ('to death').