Hi. I'm 57 and started riding last June. I smoked until I was 49 and ended up with a (very?) mild case of emphysema. For years before I started riding I was very sedentary and, though not overweight, in terrible condition. My first rides had me exhausted after a mile or two; now I ride about 15 miles an outing.
1. What type of bikes do 50+ folks ride? Rocky Mountain Metro 50D: alum frame w/ flat handlebars as high as I can get them, carbon fork, Deore gearing, hydraulic disc brakes - 27 pounds as she stands.
2. Do you take any special supplements/vitamins/minerals as you have grown older? No.
3. How much do you feel you have "slowed down" if any? 
Compared to what? (See above.)
4. How long have you been biking? New, around a while, biked all your life? See above. I biked as a kid. I shudder now at the dangerous things I did. Stopped riding when I went to college.
5. What type of biking do you do? Long rides, touring, short rec rides. The idea for me is to work on my cardio-pulmonary endurance - to get my heart rate up and get out of breath and stay that way as long as my legs can hold out. I ride a gentle uphill slope until I tire, then turn around and re-trace my route. I happen to live in a spot where I can ride gently and steadily uphill about 12 miles. (So far I've not made more than about 8 of those.) I average 10 mph for the whole ride.
6. How often do you bike? Your approximate annual mileage? I ride between 50% and 70% of days. I haven't ridden a full year yet and my distance keeps increasing. In calendar 2013 I'll probably ride 3500 to 4000 miles. Maybe more.
I've been lurking here a bit since I started riding - learned a few things. Though I'm no expert on any aspect of cycling, I am a bit of a rarity here since I'm no athlete, no racer, in lousy condition, unable to ride very far or very fast, and willing to admit it. Maybe I can contribute a slow-poke's perspective now and then.