Dic: In one's prime, In the prime of life, Past someone's prime

in one's (or its) prime: Fig. at one's or its peak or best time.

• 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.

in the prime of life: Fig. in the best and most productive and healthy period of life.

• The good health of one's youth can carry over into the prime of life.
• He was struck down by a heart attack in the prime of life.

past someone's (or something's) prime: Fig. beyond the most useful or productive period.

• Joan was a wonderful singer, but she's past her prime now.
• This old car's past its prime. I'll need to get a new one.

..... http://idioms.thefreedictionary.com/prime