。
She's at the height of her youth right now.
Literal
She [topic-は] now [subject-が] youth 's prime is.
The pattern X は Y が Z だ singles out 'now' (今) as the time at issue, marking it with exhaustive-listing が — 'it's specifically now that is the prime of her youth.' This kind of '今が~盛り' frame is common in admiring or wistful descriptions: 今が一番楽しい時期 ('now is the most fun time'). 青春 ('blue spring') is a poetic Sino-Japanese word for youth, drawing on the old Chinese five-element symbolism where blue/green corresponds to the springtime of life. The slightly literary register suits the elevated subject.