If you have skills that require technique, knowledge and thought rather than strength and stamina, fine. It really does depend on the job as well on your health. I know a number of teachers at various levels, craftspeople, people running their own businesses who will go on until they drop. But as Ariadne says, if your job requires target hassle and macho breakfast meetings and boring travel, what is the attraction? I know people who, while still fully able to carry on teaching or doctoring or whatever have lost heart and enthusiasm to the point where they go off and do their own thing.