There are relatively few science fiction or fantasy books with the main character being an old person.
My personal feeling about science fiction is that it’s always in some way connected to the real world, to our everyday world.
It may be far in the future, but there’s some kind of logical way to get from where we are to where the science fiction is.
I love biomedical science, I love astronomy, and you can’t really do much with those in a fantasy setting.