Tough question, very similar to the one about when you should hang up the car keys. If you leave that decision to the individual involved, like we do with automobiles, I'll expect to see a lot of 80 or 90 somethings weaving along the highway on their Harley's at 20 mph. It is just human nature to see your own capabilities through rose colored glasses, even as you sit in traffic bitching about the other old geezer holding you up. I'm only 60, but even through my rose colored glasses I can tell that my capabilities have been significantly diminished. Eyesight and reaction times seem to go first, and you can compensate for those things by riding more conservatively. Physical strength is another thing that erodes slowly so you don't realize it's happening until you find your leg doesn't support that 900 pound bike at that off camber intersection and you lay it down. A strength training regimen at the local gym can help with that. But eventually, the mind gets a little fuzzy, and then how do you tell yourself to hang it up? In many cases there will need to be outside intervention, be it family members or better yet licensing agencies. I'm a firm proponent of increased testing requirements for elderly drivers, and that should include motorcycle testing for those with motorcycle endorsements.
As for voluntary methods, I like VaEagle's idea of doing the riding course on a regular basis (yearly, every two years, ? ), to get an unbiased opinion of your skills. But that method requires removal of the rose colored glasses, and it still requires you to have the mental ability to make the correct decision when those skills are no longer adequate. So it comes right back around again to outside intervention in many cases. Another slippery slope issue.
Jerry