godan verb (-gu), intransitive verb
act young; look young
Describes someone behaving or appearing younger than their age, often in a positive or admiring way. Can be used for both actions and appearance.
彼女はいつまでも若やいでいる。
She always acts young.
その服を着ると若やいで見える。
You look younger when you wear that outfit.
若々しい is an adjective meaning 'youthful' in appearance or spirit, while 若やぐ is a verb focusing on the action or state of being/looking young.
Derived from 若 (waka, 'young') + やぐ, a verb-forming suffix meaning 'to become like' or 'to act like'. The suffix やぐ is related to やか (as in 静か) and やぐ (as in 賑やぐ).