。
She's at the peak of her youth right now.
Literal
She [topic-は] now [subject-が] young prime is.
若い盛り ('peak of being young') uses 盛り as a noun-modifying suffix, parallel to 男盛り, 女盛り, 食べ盛り, 働き盛り — the suffix builds 'peak X years.' The bare i-adjective 若い modifies 盛り attributively, a subtle structural variant alongside the noun + の + 盛り pattern (e.g. 青春の盛り). Both build the same general idea but with slightly different grammatical bones.