I recently read this great article about Reagan's change in speech pattern. It raised the question if he had early onset dementia while in office.
Early onset dementia is usually defined as affecting people younger than age 65.
Reagan was three weeks from his 70th birthday when taking office. Reagan died aged 93.
Therefore, it is highly unlikely he had early onset dementia.
Various articles make the claim, however the original statement that the articles are based on seems to imply that it is a matter of predicting the future onset of Alzheimer's based on subtle differences in speech.
The findings, published in The Journal of Alzheimer’s Disease by researchers at Arizona State University, do not prove that Mr. Reagan exhibited signs of dementia that would have adversely affected his judgment and ability to make decisions in office.
But the research does suggest that alterations in speech one day might be used to predict development of Alzheimer’s and other neurological conditions years before symptoms are clinically perceptible.