Dic: in someone's prime, prime of life

■ prime: 7. a period of power, vigour, etc., usually following youth (esp in the phrase the prime of life)

■ 4. [N-UNCOUNT] usu poss N △If someone or something is in their prime, they are at the stage in their existence when they are at their strongest, most active, or most successful.

• She was in her intellectual prime...
• We've had a series of athletes trying to come back well past their prime.
• ...young persons in the prime of life.　
• She's now forty and still in her prime. 
• He is now past his prime. 
• a man in the prime of life 
• a young singer who was tragically cut off in her prime (=died while she was in her prime)

..... Collins, COBUILD, LDOCE