Each deity has a human avatar.
If all her family/fellow deities have older looking avatars, then why don't have a younger one?
And what better female avatar for the Queen of Darkness than being such a seductress to power and influence over mankind. Who is always looking for the best to server her, and that includes her own looks.
You'd think with all that power she could hire a decent stylist and make-up artist. Sad to be outdone by rando-5th level adventurers wearing full gear. ;-)