For Norse gods, the "core" of eternal youth was found in these fruits supplied by the goddess Iduna
FIGURES OF NORSE MYTHOLOGY: IDUN The goddess of 
eternal youth,... [DOC] [TLE] Idun - goddess of apples, spring, youth & eternal life, Bragi's wife ... [PAR] Idun-Norse Goddess Idun, She who renews, is known as the goddess of spring, 
... Idun/Iduna ~ ... god of poetry and song in Norse mythology, Iunn his wife, is a 
goddess associated with youth.