Everyone experiences deterioration in their motor and cognitive abilities after the age of 40. There is nothing abnormal about his behaviour. After 40, the brain shrinks by around 5% per decade. The first skills to be impacted are motor skills. However, the same brain deterioration also affects language, judgement and impulsivity. Trump’s behaviour has nothing to do with mental illness. He is simply suffering from the normal symptoms of ageing.
You cannot come to an accurate diagnosis of Donald Trump's mental state from limited public appearances and tweets. It is impossible to reach a firm conclusion from an individual's public life alone. It is unfair to speculate about someone's mental state without conducting a psychiatric evaluation.