Yes and no. The clear signs of Alzheimer crop up usually 2-3 years prior to the complete and total collapse. You cite Reagan, but he is actually a good example of this. He was showing the early signs well before he went absolutely off the rocker. Biden showed none of those signs, at least publicly, until the very start of his campaign.
Alzheimers can really be seen as two distinct periods. The Start and The Collapse. The start has minor symptoms and usually lasts a few years. Minor drops in memory, disorientation, occasional difficulty speaking, etc. The brain is damn good at covering up for the worst of the symptoms, but clear signs do crop up and build up to a certain point then just... holds steady for a while.
The Collapse is what you refer to, the stage of varying but short length where the brain simply stops being able to compensate and everything goes to hell.
Biden had not been a recluse prior to his campaign, in fact he was fairly public as a persona. While I can't dismiss the possibility that The Start was covered up either deliberately or accidentally due to Biden's mannerisms... it doesn't seem likely. Instead it seems he had a Start of about... 3 months then went straight into The Collapse.
Edit: That being said, could be something like Creutzfeldt-Jakob disease. Alzheimer itself is being used loosely here since its the closest symptom wise and is the most common neurological disease of this category.
