We are biological machines.

I do not think that older people "forget" why they went in to a room. Rather, they know why but the simple preponderance of garnered knowlege from a long life fills up the memory strata. The memory banks packed with those thoughts and memories become full of all they have known and experienced, what they have felt, those they loved, those they dreamed of, every touch, smell, sight and fantasy.

Why remember you have been looking for a cotton bud when your long dead father's advice, long redundant, is in your head, or a long ago kiss, or the aromas of a summer evening with someone you loved live there? No wonder the older we get the chances of reaching overload set in.

Perhaps dementia, senile decay or forgetfulness occur only because we have taken in all that we can in one lifetime. Perhaps our strata are packed full and we would have to delete much to gather more information and/or experience. We have no interface as computers do to delete unwished for memories. Every memory good or bad stays with us no matter if unwished for all our (hopefully) long lives.

If this is is not the basis for a good sci-fi story then I am (was going to enter a long diatribe there but resisted) wrong.