in (one's) prime
In or during one's happiest, most successful time; in the period when one has the most energy, vitality, and potential. My father was in his prime when he was diagnosed with cancer. Back then, I was in my prime—if I was ever going to have a shot at the big time, that was it!
in one's (or its) prime
Fig. at one's or its peak or best time. Our dog—which is in its prime—is very active. The building was in its prime back in the Fifties, but it has not been well maintained. I could work long hours when I was in my prime.