My grandfather rode his bike pretty much all the way to the end. He was old - but I respected him more than anyone for his ability to keep going no matter what. He did not ride any event rides or pack lumber home from the hardware store via bike or even commute via a bike. He did it for the pure freedom and health of it. What kept him from enjoying riding his bikes ALL the way to the end though was this: he simply could not ride it any more due to old age and being afraid of falling over and getting hurt.
I wonder if he had discovered recumbents and recumbent trikes at an earlier age (like in his 60's and 70's) he would have rode longer. Of course, there was not much in the way of recumbents or trikes even 10 years ago outside of home builders and very very small niche products. Today the options are many and the quality and engineering of recumbents today is superb.
Right now, were are at the very front of witnessing a new wave of old. More like a tidal wave, actually. Over the next decade there will be a massive amount of Baby Boomers hitting retirement and subsequently their "golden" years. Personally, I do not think that we as a society have fully grasped what this will mean for the economy, healthcare, etc - but I think that it will be good for the recumbent industry - in more ways than one.
As people age - if they desire to keep up with their physical activities, will they move to recumbents when riding a diamond frame becomes too uncomfortable? We will see but I imagine the prospects are good. Trikes in particular will benefit. As a Gen-X'er, I have already flipped to the "dark side" of recumbents and won't likely go back, so I am a recumbent rider for life now. Unlike the previous generation before me I am starting from recumbents - not moving to them due to some physical ailment or other reason to "force" me off of a diamond frame. Likewise, my own children will have 'bents - my oldest already has a used Bike E that has become the primary bike of choice - will she "go back" to a diamond frame? Time will tell - but a very likely trend going into the future will be people starting on recumbents, not the other way around.