The mini-bio on her Profile Page lays it out in detail, but Mary Dahl (AKA Baby Doll) is a character who canonically has a form of pituitary dwarfism. She's not "a 5000 year old wizard in the body of a young person" but the fact that she can pass as a toddler or young child is a big thing in both episodes that she appears in.

I'm not sure if I want to app her officially, but I also don't want to cause trouble on your Test Drive Meme.