I remember hearing an interview with EC on BBC radio when he was coming up to 100. From the way he talked you could easily have believed ue was nearer 40 or 50. Only one question left him speechless - and that was "have you made any plans to stop working?"
you can't do many things at the age when your youth antenna is already down
You can complete a PhD when you are 76, if you want: http://www.ncl.ac.uk/press.office/press.release/item/1026487332 [Broken]
It was just his hobby, many people too have their hobbies. An IT professor I know a little about holds a PhD in Chemistry, her life is all with IT, wonder what does her ChemPhD is being used for. Higher education offers a higher level of abstraction which I think might also be acquired during practical working experience and time. Bill is an example of encouragement, "people can build a company from $100" is another, etc but would we then be our social selves and nice with each other and think of life much harder or easier; the truth remains, relativity works better as always.
When you're retired and you happen to evolve into tribe head, (pater familiaris), you've lots of family issues to attend, kids to help out, grandchildren to babysit, distant relatives to visit and help out with building houses, or cutting wood, hiking in nature to collect the nicest pictures and visit earth science projects to help digging and discovering.
I'd love to but I'm not sure if my physical situation would let me do that!
It depends what you mean by work. I would never be a wage slave but do inspired things every day. I retired from making money early in life and just am a lifetime learner and volunteer. Death is part of being human and nothing to fear or worry about. I do not think I would want medical parts replaced as it is not natural life and death. I am more a holistic individual not one that needs goals to have a quality life.