The clearest sign that Trump has declined mentally is that he used to speak coherently and intelligently, used to be able to form thoughts into entire paragraphs that stuck to one topic. Now, he speaks incoherently, jumps from topic to topic, constantly interjects non-sequiturs into his speech, and generally presents as a person that does not have total control over his mental faculties. It's not "bias" to observe those things and wonder if there is something underlying wrong with him. Perhaps there isn't, but he surely appears to have gone through a pretty steep mental decline from 30 years ago, and he certainly is not of an age where you can just chalk it up to natural aging