V3ra We have a friend, never the most sociable of people, but blessed with a very social wife. Once he retired he retreated into a chair in the living room and read. he would take his wife anywhere, she did not drive, but he never did much or met people.
Early last year he was diagnosed with mild dementia at 75, His wife died suddenly from a stroke in July and within six months after that he deteriorated so fast that by Christmas he was in care. We have visited him, but he doesn't know us from Adam, yet we have been good friends for over 40 years and has no memory of his past life, so after talking to their daughter we go every few months and stay 10 minutes.
He was a highly intelligent computer academic, I cannot but feel that if only he had been more willing to interact with other people and had at least some activities outside of the home that he would neither have developed dementia so young , nor had it accelerate so fast.
I think it is far to easy to say 'Well, I have retired, hence forward I will just do as I like. You wouldn't say now I have retired I can eat what I like and live on nothing but jam doughnuts despite weight gain, and health problems. You need to look after your mental health as well as your physical health.
All that is needed is low key events that take you out of the house and interacting with other people once a week. When energy prices rocketed, our church opened a 'warm room' once a week. It now does lunches as well. It is well used and is relaxedly sociable, do an exercise class. I do Tai Chi and we then go out for a cup ot tea or coffee. Things like that are all thatnis needed.