Dad was diagnosed in October 2017 aged 87 although I had made my own diagnosis of alzhiemers at least five years earlier. He died aged 89 in February 2020 If he hadn't have had cancer then he would probably lived a few years more.
His brother made almost 95 and there is a formula based on ages where a person with dementia is expected to live half as long (after diagnosis) as a person without dementia which would have given my dad a few extra years to about 92 or thereabouts.
It is too complicated and it only takes a simple thing such as a small infection to upset the apple cart and bring the expected lifespan down by a number of years as in my dads case. I fully expected to still be caring for dad for another 2 or 3 years at least.
There is just no knowing.
