WordNet (r) 3.0 (2006):

prime of life
    n 1: the time of maturity when power and vigor are greatest
         [syn: prime, prime of life]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "prime of life":
   adulthood, adultness, age of consent, bloom, budtime, driving age,
   early years, florescence, flower of age, flowering time, full age,
   full bloom, full growth, fullgrownness, grown-upness, jeunesse,
   juvenescence, juvenility, legal age, legalis homo, majority,
   manhood, manlihood, mature age, maturity, my burning youth,
   my green age, prime, ripe age, riper years, salad days,
   seedtime of life, springtime of life, tender age, tenderness,
   toga virilis, virility, womanhood, womanlihood,
   years of discretion, young blood, youngness, youth, youthfulness,
   youthhead, youthhood, youthiness