Her father Ivalde, is a giant and Star-Hero. One can assume that the giants were the oldest family of gods; and that the new gods superseded them. This made many of the old race of gods either demonized; or absorbed like Skadi and Gerd, who became part of the new order of gods.
Idun is usually portrayed as a sweet maiden. In some stories she is shown to be naive and easily led astray by Loki. She is always shown to be very young, and she carries a basket of filled with golden apples.
She is responsible for the health of the gods, because without Idun's apples the gods would age and die. Idun's apples are indicative of prolonging life rather than immortality. We know this because the gods have to keep eating them.
Idun is definitely the goddess to invoke for keeping a youthful appearance. She would also be a good one to invoke for vibrant good health.
Invocation for Idun:
Idun, Asa-Goddess of long life,
Keeper of the wondrous golden apples,
Grant me the youth and long life of the Gods,
Youth and enduring life of the mind as well
as of the body.
Let my keyword be flexibility,
That my expansion in the realm of magic
May be unending.
May Idun Bless,