。
She has kept her youthfulness well.
Literal
She [topic-は] youth [object-を] sufficiently is-preserving.
若さ (youth/youthfulness) uses the nominalizer ~さ to turn the adjective 若い (young) into an abstract noun. 保っている (is preserving) with ~ている marks this as an ongoing maintained state. 十分 (sufficiently) indicates she's preserved her youth to a satisfactory degree — she looks good for her age.