I assume a lot of this has to do with the deterioration of health. When I adopted kittens, the shelter paid to have them "fixed" (sterilized) for free but one was slightly too young for the procedure. A couple of weeks later, he got the surgery and his personality changed drastically for about a week or less. I was afraid it would be a permanent change, but he clearly just didn't feel good and that cleared up with time. I assume aging is a slower version of that just based on observation of older family members and the limited decades that I've experienced firsthand. If you feel physically bad, of course your attitude will change.