The Norse goddess who keeps the apples of eternal youth is Iðunn.
Iðunn is associated with the apples that maintain the youth and vitality of the gods. In the Prose Edda, these apples are essential to the Æsir, who begin to age when Iðunn is taken away. Her role makes her more than a simple orchard keeper: she safeguards a resource on which the gods' continued vigor depends.
The best-known story involves the giant Þjazi. After forcing Loki to help him, Þjazi lures Iðunn outside Asgard, where he carries her and her apples to his realm. Without the apples, the gods grow old and grey. Loki is compelled to rescue her, transforming her into a nut and carrying her back while Þjazi pursues them in eagle form.
Iðunn is commonly confused with Eir, a goddess associated with healing, or with Freyja, who owns other magical treasures. The apples are not the Mead of Poetry; that drink comes from Kvasir's blood and grants poetic or intellectual ability.