Been declining since before his frontotemporal lobe dementia symptoms started to show in public in 2014 - just go back and look at the pictures of when he started to stand with that curious forward Lewy Lean stance, and when he lost fine motor control and had to start drinking from a glass with both hands.
Not unusual for the USA, Reagan had full-blown Alzheimers since his assassination attempt and served 6 years with it.