I'm 49 and my knees are feeling the abuse I've given them over the years. I started riding fixed again last year after a long layoff, and my own subjective experience is that I think my knees are stronger and less painful now than before. After a few thousand miles fixed and a few more freewheel, my knees are strong enough that I'm running again -- probably a stupid thing to do, but the point is that they hurt too much to run on them a year ago. I don't know if riding fixed helped or not, but it seems to me that it has.

There are a couple riders in my area that are amputees above the knee. They are fast, too. (Freewheel). I can't comment on prostheses and fixed gear, but I would think that riding fixed might put additional stress on a prosthesis.... i.e. a pulling action when spinning fast or trying to stop. Something to consider anyway.