Sorry. I first assumed you were joking, but then I re-read the context, and I gave your sense of humor the benefit of the doubt. :P It’s not wholly unprecedented for professors to keep teaching to quite an advanced age. Harvey Mansfield, for instance, whose class I took a couple years ago, is 78.

I spent quite a bit of time reading up on Donald Knuth, have his books on my shelves here (second copy, first one got misplaced during an emigration).

He's a very interesting man with lots of depth outside of our field as well, especially in music.

The retirement was sort of half way because I think he really is pushing himself very hard for someone that advanced in age, but I can see how he would like to complete his magnum opus in the foreseeable future.