Fetching details from the dictionary...

youthful; young; young-looking
祖母は八十歳だが若若しく見える。
My grandmother is eighty but looks youthful.
若若しい声で挨拶する先輩が好きだ。
I like the senior who greets people in a youthful voice.
運動を続けていると若若しさを保てる。
Continuing to exercise helps maintain youthfulness.