Did Retirement Accelerate Cognitive Decline?

My dad was actually a great scientist. So lots of people were eager for him to do part-time consulting. He said no to every single offer, and my dad then did decline mentally. We found out later that he had Parkinson's disease. Did retiring accelerate cognitive decline or did my dad have a sense that he was cognitively declining? And therefore, did that make him retire? Or maybe somewhere in the middle, which is he was experiencing some cognitive decline which made his work less fulfilling slash rewarding slash successful. I think all stories are plausible and likely.