Prime Of Life
noun
Your robust years are the best years of your life! This is the prime of life.
そうねん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼のお兄さんは壮年期に心臓発作を起こして亡くなりました。
His older brother had a heart attack and passed away in the prime of life.