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(Aside: There is no allegation here that Biden has the early onset type of Alzheimers.)

Normal dementia (something I have seen up close) is usually not an instantaneous switch. I have watched it slowly take down someone of the course of 10+ years, and the early stages are just weird comments here and there. Then maybe things are remembered that are 80% true (e.g., finding a note written twenty years ago and thinking it was written yesterday and it needs to be acted upon immediately). Maybe a thing is remembered as happening, but the background behind it is not instantly recalled, and the person fills in with other information to make it seem sensible...after all, they can still be coherent much of the time.

During the early stages, the person can seem totally coherent at times, but only have spells that cause people to wonder...privately at first...aloud later..."How can grandpa think that?" is often the first reaction, but nothing more. It may be years before an event happens that causes someone to say, "Oh my god, we've got a serious issue on our hands. Grandpa needs to see a doctor."

People make excuses for (or just ignore) oddities until they pile up and a need for a medical diagnosis is staring them in the face, and they can no longer deny it.

Some people continue making excuses even after the need for medical treatment is obvious.