If you're seeking some martial-arts inspiration today -- something to get you back into training or to keep you going as a 50+ practitioner -- check out this video of taekwondo Grand Master Kyu Hyung Lee demonstrating 8th Dan form Hansu.  Notice especially the gray hairs.  This Grand Master is one of us.
   Obviously this is a man who has devoted his life to taekwondo, and the rest of us mere mortals -- born with either less natural ability or less drive -- can't expect to sustain this level of proficiency when we're 60, 70, or 80.  But we don't need to.  The simple rule of thumb is "do whatever you can with whatever ability you've got."  Practicing your martial art will improve your attitude and your health, even if you have to work around arthritic hips and diminished stamina. 
   If you're just getting back to the martial arts after a long layoff, begin by seeing your family doctor.  Also consider working with a physical therapist to help get you stretching the right muscles and joints the right way.  It's quite possible that stretching techniques you learned, say, 30 years ago are no longer considered state of the art. 
   Okay, enough talk.  Let's watch Grand Master Kyu Hyung Lee in action.  http://www.youtube.com/watch?v=u_DRKUJsNCk&feature=endscreen&NR=1