William , before I respond to your question, I must ask ,Your profile says Glasgow , is that UK or somewhere in the US?
We started later than some of the others. We were in our 60's. We married later in life, and it was not until then that the last of our children had moved on.
We had always been of the mindset of there being a time for every purpose and up until that point the purpose was seeing them off into the world.
We had always maintained an active sex life between us and had , early on, gotten into the habit of reserving time for just the two of us to do things together outside of the home. We realized that having offspring around was only a temporary thing.
When we had the empty nest the questions were :
What's next?
Why not?
There were some personal elements for me as well, but I have addressed them in other posts.
For the two of us it was adventure.
Because of our longer time together, right around 30 years, and our ages we did not have many of the issues some face. We knew each other and had a bedrock surety in our relationship.