When do people usually get ‘dementia’ - IF they’re going to get it?

My next door neighbour died at 93 i was speaking to her 2 days before she definitely didn't have dementia, she had slight decline i would say, but i don't think dementia is inevitable. Dementia runs through our family on my mums side i do think in my limited experience it is genetic,

Any time OP. I know people who started showing signs in their 50s, and some who started showing signs in their mid nineties.

I am afraid that it doesn't work like that and the risk of dementia increases with age:
"The risk of Alzheimer's disease and other types of dementia increases with age, affecting an estimated 1 in 14 people over the age of 65 and 1 in every 6 people over the age of 80." www.nhs.uk/conditions/alzheimers-disease/

However you have said your mother has become housebound over the last couple of years. This could also increase her chances if she is not continuing to be mentally and socially stimulated.

At the end of the day, worrying about and looking at the likelihood of her getting dementia or not isn't going to change anything. She could live until she is 115 in good health, she could gradually develop dementia over the next year, she could drop dead tomorrow, as could you, as could we all. There is no point in worrying about problems she doesn't have.

Focus on what is and not what if.

Data suggests that risk doubles every 5 years from 65 to 90. Less research in the 'oldest old' (term often used for 90+) but the data there is suggests that risk continues to increase.

Early onset Alzheimer’s or vascular dementia can happen quite early. Cognitive decline happens usually in older age. A lot of people die before it happens. If you live into your 90s you are likely to have decreased cognitive function. They may not label it dementia but the effect is the same.

MIL was 92.5 and had no previous signs at all
The first indication was that she had problems operating the microwave and the TV remote

My dad was in his 80's and started to show signs after sustaining a head injury. He was previously very active and engaged in local societies, gym 4 times a week, out every single day, good supportive friendships, etc. Obviously, it's difficult to blame everything on the head injury but that did seem to be the start of it to us - he was never quite the same afterwards.

ETA - He did actually receive an Alzheimer's diagnosis.

Mil was in her 60s

I'm not 100% sure but I do work with the elderly so I have some insight. Not all the elderly get diagnosed with dementia however a significant number do have some cognitive decline. This isn't necessary dementia. Its great that your mother has very sharp memory and long may it continue. I do come across some very healthy and sharp 90 year olds. I think a lot depends on how active and involved they are with the world around them.

My mother was showing a few signs and then diagnosed with a rarer form of dementia when she was almost 92.

My two grandmothers never showed any signs (died aged 89 and 94).

I don't think it's something you can predict really, but it is definitely an old person's disease and the increase in the number of cases can be explained by our ageing population.