So what?
I am an academic. Used to think I’d retire at 62, but now I’m thinking it might be between 65-72. Not because I have to work, but because I enjoy working. Also, I’ve noticed that some people (not all) suffer significant mental decline after they retire—including those who retire relatively early.
The critical thing is: Do the students find your course engaging & valuable? I teach computer science, so its also important to keep up with new developments. When you can do longer do that, it’s definitely time to go.
My company has no retirement age. I've worked with many colleagues in their 70's that are still sharp and productive. Many have outlived a spouse and enjoy work for both the social contact and mental stimulation. Many are fountains of knowledge that has not been imparted to younger colleagues to carry the torch. I've tried to do that, but my customers keep running contracts off the rails. The junior employees have to hit the bricks to keep the bills paid. I expect my career ends with a face plant on my keyboard. That seems to be the norm in my business. The hardest part is cleaning out the office of a deceased colleague.