prime of youth; bloom of youth
Refers to the peak period of youth, full of energy and vitality. Often used in literary or slightly formal contexts.
He is in the prime of his youth and devotes himself to sports every day.
I poured the energy of my youth into my work.
Compound of 若 (waka, 'young') and 盛り (sakari, 'peak, prime'), with rendaku voicing of 盛り to ざかり.