There are assisted living facilities in Thailand, most are focussed on people with special needs, rather than just old. I.e Dementia, etc. Traditionally and I generalise of course, Thai culture works on the principle that the aged stay and live out their final years with the family. Plonking your aging and annoying parents into a old peoples home, is somewhat offensive and distained in Thai culture. The aged are the responsibility of the family.
The question you need to ask is the escalating medical costs you will experience as a you age. You are not and never will be eligible for what little National Health exists in Thailand, and to get the standard of care that you as a westerner would expects will cost, a lot, as its all private. As a foreigner, you can have it all in Thailand, as long as you can pay for it.
I have many expat friends, approaching their twilight years, that are doing strong in Thailand. They have secured good insurance, have in home nursing and care (obviously very cheap compared in Thailand) and , above all, a caring partner (often a Thai National) that will ensure that they get the care they need. In thailand you dont have a government system that is actively protecting your well being.
For the record, I am not in anyway criticising the Thai Health System. Thai Hospitals, Doctors and Nurses are some of the worlds best. But, as a low income foreigner, you are not guaranteed of access to Thai Public Health. Your medical care will only go as far as your insurance cover.
My point is this. Thailand is cheap for a expat to live, as long as they live as a local. But personally, when I get old and obnoxious, I want to be in an Australian Hospital, with full Medicare, and a pension. With a government welfare officer that speaks English, at my beck and call.
Whatever you decide, do your sums well. There is a growing problem in Thailand of penniless foreigners, homeless and adrift in Thailand.