Wow, I'm 64, raced two scores ago, and find this thread both inspirational and depressing. Interesting that with age some are more confident and some are more cautious.
I used to climb up a 26 foot ladder to fix the gutter. Soon after turning 40, I would get about twenty feet up, then a funny feeling would take over. I would start thinking about the mortgage, and how would the kids get through college, and the legs would start to shake. The older I get, the less I like ladders.
By age 50, trying to get back in shape, I was pretty shaky doing descents on my old 70's bike. I would pump the brakes to stay under 30 mph. But after getting a new bike in 2006 with decent (rock solid) wheels, tires, and brakes, I gained alot more confidence and now feel comfortable descending at "terminal" velocity, which is mostly ~37 mph on the downhills from the bluffs around here. That assumes all the precautions expressed in many excellent posts; not "getting too far out over your skis".
One recommendation (from another thread) that really helped me was to bend the brake caliper springs a little, making the levers much easier to press and modulate the brakes with arthritic hands.
We have a 400 foot, mile long hill with 20% grade on part of it. That descent accelerates me to just over 50 mph. For a minute or so my 215 pounds (65 of which are "extra" pounds) turns from a health liability into a worthy asset !