This will go down in history as an all time classic.
"There are known knowns; there are things we know that we know.
There are known unknowns; that is to say, there are things that we
now know we don't know. But there are also unknown unknowns –
there are things we do not know we don't know."
The old horse in the stable still yearns to run. (an Asian saying.)
This means people with much life and work experience, especially those who have retired, still have unfulfilled ambitions and accomplishments yet to be achieved.