Are there any facilities where younger patients with dementia who are still high functioning can be placed with Medicaid and Medicare?

Your sister has Medicare? Was she able to get SS disability for her problem.

Sorry to say that NHs and ALs lean to the care of the elderly residents. There were a couple of younger ones at Moms NH and I felt sorry for them having to spend their days with Dementia and very sick older residents.

Maybe a board and care where there are less people and more homey.

I'm afraid I have no useful suggestion to provide other than I recall reading a few years ago that there are nursing homes (and perhaps other caregiving facilities) in which one wing may be dedicated mainly to younger residents, who are listening to popular music and ordering pizza.

You'll probably have to ask around for this. I would think that, despite the apparent contradiction, an Area Agency on Aging service might provide some insight on this as they should have some information on the local facilities.