Big Pine, you should go try it yourself first on a mountain bike. I did a lot of years ago. Granted my bike had zero suspension, but I remember how incredibly rough and rocky the descent was. We walked/pushed the bikes to the summit. And then one of the bikes blew a tire on a sharp rock on the way down, so we had to walk it out.

I cannot imagine a couple of people in their seventies who aren't in very good shape managing to climb or descend that White Mountain with a mountain bike. They might be able to do the first 5 miles where the road isn't as bad, but that final climb....