That's the hard thing to decide. My father a brilliant psychiatrist obviously always had a pretty good mind and was wise with money. Also dementia is not instant. It can 10 years in the coming. My mother said even before she died (she died first) his brain was going (long before any diagnosis).
I think just having as much contact as you can helps. A sibling and I went to see our father when his other carers (who might have been making it up) expressed concerns. Our father could point to the sum he wrote in his diary when his bank statement came in and said he had lots of money (true at that point) - so he was mentally agile enough to take a bank statement and write the sum down in the diary. He did not seem to understand that the fact the money friom a few months before was a lot higher might mean too much spending (and not by him). However we wanted him reassured and he wanted to be there in his own home until he died (he was and it cost with no fraud £130k a year for day and night care in that last year - his solicitor said it often is £100k so that was not too out of line). So we left things as he wanted them, accepted the will change (this person got 25% which is obviously not 100% so not too bad) and had a little battle over registering his power of attorney (which was in our - the children's favour set up years before - we never ever fell out with him right to the end) which took over a year or more and he or those controlling him (which we will never know). Then he died so the problem went away,.
Even things like checking his finances - he kept saying you can look at anything but the filing cabinet was locked and he did not know where the key was and "the person" had it at her place but would not answer calls to get it to us. Then people with dementia often say one thing to one person and the opposite to someone else. His lawyers would not let us check the bank statements but instead hired his accountants for £5000 (!!) to do what we would have done for nothing. They found after some months nothing huge had gone out like £100k, just the various supermarket trips etc so we left it and it was not so bad. It was nothing like the person above where every penny is stripped away.
We were very aware of that difficulty - when is someone allowed to be in charge and when not. I want to be in charge until the last and after the trouble over my father's power of attorney I don't want to execute one so I'd better have my mind until the last as my mother did.
Had we done it again I would have visited more and been more of a presence in the house (which my father would not have minded at all). I would have tried to see his doctors with him rather than having no information about his care. However it's hard if you all have small children and all live a long way away and all work full time - that was the positoni of all 3 of us children. I still don't really know if there were any financial elder abuse but I do know that had he been in his right mind our father would have left his money (not that there was that much left anyway) to his children equally rather than 25% to this other person who was hardly close to him. It was not as if they had been a full time employee or lover or anything like that. I suspect he offered and it was accepted which is interesting. He offered a large sum of money to one of us and we said of course we don't take it - you need it but an out sider would just say yes and as this person who was still operating their own affairs had offered it that is not necessarily illegal - just unwise.
Bettencourt that rich US lady died aged 94. She ended up perhaps financially favouring someone too much in her older age and it went to court. It is a huge issue around the world.
On the one hand I feel perhaps people should make their will before age 70 and not be allowed to change it but then they would still give money away to carers and yet some people are totally on the ball until much older. My children's last grandparent published his first technical engineering book at about 86,