There's been some good discussion elsewhere on the forum about how old Jadis/the White Witch should look onscreen and whether the actresses who have been suggested for the role are younger than expected.

Personally, I never pictured the White Witch as being particularly old. Mind you, I don't see her as being a teenager either. Mid-20s to lower 30s is what I imagine. (According to what I've read the actresses who portrayed her in 1988 miniseries and the 2005 movie were closer to 40 but I couldn't really tell that with all the makeup they had to wear. I don't mean that as a criticism! The book's description of her skin color demands a lot of makeup.) I admit the reason for my assumption is that C. S. Lewis stresses her physical (if unearthly) beauty, and we normally associate beauty with youth. I feel guilty about that. Older people can certainly be beautiful. But I feel like the association of youth and beauty is so ingrained in us that if C. S. Lewis meant to indicate an older beauty, he would have been more specific. 

Maybe the reason some fans picture her as older and feel that the actresses suggested have been surprisingly young is that the White Witch is usually frowning in the illustrations which gives her wrinkles.