edit: Sorry about the word-soup title. Totally missed that. "How to write characters with disabilities… without anachronisms?"
A couple of things, here. First, disabilities can mean a wide number of things but in this post I am talking about the developmentally disabled. Consider those with autism, Down's Syndrome, or intellectual disabilities (i.e. mental retardation, though we don't use that word and for good reason.) I want to write a character who is mostly non-verbal, who is severely impaired in brain function, but who is still capable of fine motor skills and the basic necessities of living. She is a main character, and I want her to be a wizard. Wizards use their powers to change the world from the way it is into the way they think it ought to be; and I thought that, since people with autism see the world so very differently than the rest of us, what would it be like to have a wizard with autism?
My problem comes when I consider how to describe the disability without using anachronism. If this is your typical "medieval fantasy" story, words like "autism" or "disabled" or "mentally handicapped" will not be appropriate. (If you're not convinced, I posted a story on here about a girl who had a physical disability and she described it one time as a disability and every single reader called me out on it.) Within medieval times they might describe a disabled person's smile as an "idiot's grin" or they might describe a physical disability as a "gimp" or "deformity", and I hope I don't need to explain why those two descriptions are not appropriate either.
My sister has a developmental disability so I'm actually at a bit of a disadvantage. When she smiles, I see her smile, not the smile of a disabled person. When she claps her hands with a funny look on her face because she's happy I just see my sister, I don't see her disability. While it is wonderful (and right) to look past a disability and see a person, it is also very unhelpful for communicating that disability to the audience.
I don't want this to be subtle, I want the reader to know it from the beginning. We aren't dancing around the subject of disability. It's the whole damn point of the story.
Any thoughts on avoiding anachronism but also avoiding horribly offensive and outdated terminology? Is there some middle ground that I'm forgetting? I do think fantasy, and literature in general, would be richer if it tackled more mental illnesses, disabilities, and handicaps.