。
We're in the prime of our youth.
Literal
We [topic-は] youth [attributive-の] right-in-the-middle [is-だ].
青春 ('youth, the springtime of life,' literally 'blue-spring') is a poetic term for vigorous, idealistic youth. 真っ只中 ('right in the middle, dead center') is often used metaphorically for being deeply immersed in something. A somewhat romantic, self-aware declaration — the kind of thing said half-seriously.