FWIW, my dh often swears blind that I haven't told him this or that. The fact is, he just doesn't listen - or else he hears but doesn't register it because his mind is full of something else. It's def. not dementia (not yet anyway!).
One of the earliest signs with my mother happened while on holiday with my sister, who noticed that she started to read the same book from the beginning a few times - she had always been a great reader. She was also getting repetitive.
However, having already been through it once with FiL,,I chose to dismiss what my sister said, especially the repetitiveness, until the day when my mother phoned her bank about something - she'd always been very clued up about finances - and could not remember, literally the instant she'd put the phone down, what they'd said.
That was when the penny finally had to drop.