The people who followed him on the 1980 campaign found he was entirely capable and mentally up to speed. They knew he was 69 but they saw he wasn’t elderly yet.
It was after he got shot that he started losing it.
True, but the plaques in the brain are specific to Alzheimer’s and are not related to trauma. Maybe Reagan didn’t actually have Alzheimer’s, but he wrote that he did. Anyway, he was never a very sharp blade in the drawer for whatever reason.