This isn't what his doctors have said.

>...In 1989, just a few months after he completed his second term as president, Reagan underwent surgery to drain a subdural hematoma caused by a fall from a horse in Mexico. Such brain injury can increase the likelihood of developing Alzheimer’s disease or perhaps even cause it.

https://hekint.org/2023/11/30/the-medical-history-of-ronald-...

After this Reagan had annual exams for signs of Alzheimers and was diagnosed in 1994.