Was this in Texas? My Dad worked in a place that had been a tubuculosis hospital but transitioned to the adult mentally retarded.
There were people who had been there for thirty and forty years (since they were infants or toddlers) who had epilepsy - and had no mental issues at all (other than not having been taught a lot of things - because before 1950 or even 1960 something - epilepsy was considered equal to mental retardation).
After he'd been there a while, the transfers to group homes started to happen for every resident who was not in such poor health or so badly mentally impacted that they were bed ridden or required one-on-one care at all times. Someone who is forty and never learned to read or write past a second grade level is probably NOT prepared to handle their own cooking, cleaning, laundry, and the like.....without more training and supervision.
Reminds me of a gal I used to work with. She had some sort of muscular problem which left her unable to stand or walk on her own from the age of 6. Her body was weak, but not her mind. However, her father decided she shouldn't go to school or otherwise leave the house. He wanted her to go into an institution, but her mother wouldn't allow it.
Her older brothers had to sneak her out the window so she could go see movies and such (bless them!). She ended up in a nursing home after her mother died. She was there well into her late 40s, she never knew there were options.
But then...in the late 1980s, a young man with a muscular disorder whose family worked very hard to help him have as independent a life as possible was in the nursing home for a brief stay (a couple weeks recovering from pneumonia IIRC). He saw that her mobility problems were like his own and he was receiving a fair amount of aid from the State of Texas to have a motorized wheelchair and home health worker, etc. So he asked his caseworker to talk with her. In the end, for about the same money and state benefits that had been going to a nursing facility, she got her own little apartment, a motorized wheel chair, home health care worker to help in the mornings and evenings, was able to get into a GED program and eventually get a job. She was a sweet lady and I think that young man was an angel for helping her get her life back.
But how different her life could have been in her father (and society in general) hadn't had such "special" ideas about children with disabilities.