Working with mentally/disabled students of all shapes and sizes, I have a few things to say about this.
I remember one student who is now in high school. The student is 17. This student is probably about 160 lbs, can not speak, barely signs,not potty trained and can not walk independently. (but not confined medically to a wheelchairs) An adult stroller /modified wagon is used to get around school, which is public middle/high school. Instructionally, there is a special ed teacher for the student and another classmates with similiar disablities as well as two assistants and a nurse. I've taught in that classroom, and while physically straining, it was rewarding.
As the students age, there are some things they are more capable of. I have less severly disabled students that are now working in jobs, such as sorting clothes at Goodwill or bagger/janitor at a grocery store.
To keep a student small, its to me unethical. A person ages with disabilities or not. I've seen many adult disabled students still living at home, large size or not. Some of my kids are very large--way larger than me and if taught well there isn't a problem.
Disabled teens are a productive part of my local area. These students work, do community service hourse for school shredding papers and distributing mail--yes even that severly disabled student that I spoke about in the first paragraph has a job at the school shredding papers for the office. These teens upon graduation get jobs either at the supermarket, a warehouse or working in a trade. They know how to survive, even if they stay at home with thier own families.
I can't say there aren't problems with disabled teens/young adults. There is a whole host of delimmas that occur. Items such as hygiene, shaving and bathing is a concern and is dealt with in hygiene curriculum. Dating and intitmate relationships are a concern as well. Some female studnets would mistake attention from male students (who were just being nice and polite) as they want to go out with them. Some try to be extrememly promiscious or are promiscious and do wind up pregnant. That has happened.
I'm just saying that this child that the OP is speaking about is going to be small forever just for the convienence of her parents. There are adult sized items that can be used. Teen/adult disabled students can become productive members of the society --no matter how disabled they are. I'm amazed constantly on how many disabled kids in my area are doing amazing things... formerly nonverbal kids speak, non motivated students work at a paying job and some move out on their own in an apartment/townhouse.
With training and proper education, mental capacity can change. I can't tell you that when I first saw that student in the opening paragraph I thought wow.. this is a severe case. This student can't go to the bathroom, eating is an issue, walking is an issue, she can't speak, barely signs (uses sign instead of talking) and can barely hold a pencil...how can I educate her to become a productive member of our society? I followed the lead that was given to me. This student can work--she works in the office shredding paper for the secretaries. Even with all these disablitilies, she can be productive. I don't think anyone would have thought that she could do a function. Even as an educator, I thought the student would be better off in an institution at first.. (It kills me to say that, because I didn't know how to help the student at first. )
To rob a person of unknown capabilities just seems wrong to me. But as I see studnets grow change and learn, I see that even in very very very severe cases productivity can occur.