I think BarracksSi is right on track.
At 51 I bought my first bike in June on impulse, with no research as to whats available and the pro's and con's. It was a comfort bike, because I wanted "comfort."
And it was/still is comfortable for 30 minutes or so, but as I began getting in better shape and extending my rides I began developing lower back pain.
After doing some research here and the internet in general I decided I needed to get less upright, and since I already spent most of my fun money on the new comfort bike, I jumped on the first thing that came along on Craigslist, as not having a bike is no longer an option for me. Too much fun and good excersize!
It was an 18 year young mountain bike with very little wear. (Rock Hopper Comp.)
My first ride on it was not fun, as I was not used to the new position. My upper body and neck wore out fast---but my back did not hurt. Long story short I worked my way into mountain bike shape and now love it. I can still get some back pain but only after a long (for me) 25 mile ride.
I would love a new road style bike, but I just can't make myself ride on streets, (yet) - so the mountain bike on bike paths is fine. Like I said, I am having a blast. I am an old dirt biker at heart anyway.