Ronald Reagan was widely believed to have suffered from the early stages of dementia during his second term, but it was never (officially) spotted.
Trump has actually compared himself to Reagan saying that he, like Reagan, is having to deal with “fake news” that he has the early signs of dementia.
Drawing that comparison certainly seems like something less than genius on Trump’s part, given how widely it is believed that Reagan was showing early signs of dementia during his second term and that he was indeed diagnosed with Alzheimer’s after leaving office.