It is sad that learning a

second language at any age can help to keep our brains active and ward off dementia.
I suspect that more research is needed to actually back this statement because I do know older and once bilingual people who now have severe dementia. I also came across two more at the hospital yesterday. They were new patients in the psychiatric ward my friend is in.
One patient was an elderly Greek woman who was clearly confused. She had Greek visitors, one of whom is friendly with another elderly Greek woman I know. The visitor recognised me and, before I could go any further, started to ask me questions in a mixture of Greek and English. I took a wild guess at what she was asking and explained as best I could in simple English. At least she did not, like some older Greeks I have met, believe you can catch dementia in the way you can catch cold. The nurses looked on bemused and then relieved.
Then, while waiting for the nurses to finish "a procedure" to my friend, the new patient from the room next door came out. He looked at me in a puzzled sort of way and said,
"H-ello."
It was clear that getting the word out was an effort. I returned his greeting and smiled at him. He smiled back and wandered off down the corridor. The "tea lady" asked him if he wanted a drink. He heard her but he didn't seem to understand her at all. He stood there for a few minutes and then wandered back again. The door next to his was open. He stared in obviously confused about where he was. I also know enough to know that the occupant of that room would cause a serious disturbance if he entered her space.
So, there being no staff in the immediate vicinity, I said "No, R.... "
He looked at me. I suspect people have been saying "No, R..." for long enough that he does still understand that. I pointed to his door. He smiled and put both thumbs up and went into the room. After a while he came out and just stood in the doorway.  He seemed completely confused again. I suspect he has Alzheimer's and he may be there to be assessed for a nursing home. His name is also one which suggests English is not his first language.
Older speakers of English as second language who have dementia are a growing problem here. Even those with excellent English tend to revert to their first language. My sister's mother-in-law never had good English and, by the time she died, all she spoke was Greek. Others find English confusing and no longer follow questions or instructions well. Speak to them in their own language and they may do considerably better.
I just wonder at how confusing and frightening the world must be if you are losing your capacity to communicate. That would be frightening in itself but to have everyone around you speaking a language which is not your language would be far worse.
I was able to go in and see my friend at last. We can communicate in the same language. She is still rational and able to understand and make herself understood. That is something to be thankful for - even though she might not realise it now.
On my way out I see R... again. He smiles at me and says "H-ello."
And I smile and say "Hello" back because at least I can do that much for him.