ok, after a quick search on the web, I found these:
STAY ALIVE DEFENSIVE MOTORCYCLE TRAINING ACADEMY - Home
Defensive Driving Online Course from National Safety Council - Motorcycle
so to answer your question, "Do defensive riding courses exist?", I would have to say "yes.", but, since we don't know were you are from, we have no idea if any are in your area. that, my friend, you will have to look into yourself. but, they are out there! so there you go. ride safe.
The first one on the list is located in Buccan, Queensland, Australia.
Probably farther than you want to travel, eh?
The second one seems to have four locations in Texas, Florida, New Jersey and California.
The third seems to have courses all over the country, so they may be the easiest to access.
I am the same age as you, just turned 63. I have had a mc licence since I was 14, and rode 100 cc tiddlers out on my friend's and cousin's farms back then. Also rode my brother's 650 Honda to work for 3 or 4 weeks back in my mid 20's when rebuilding the engine in my car. Other than that only an occasional short ride until I got my first scoot, a vn750, just after I turned 55--eight years ago.
My riding experience is probably not a good one to share with your wife. Six weeks after I bought the bike I got too close to a guardrail on an exit ramp and dragged my leg for 50 feet and busted my lower left leg pretty badly. The bike was not seriously damaged, but I lost 4 riding seasons before getting back on the scoot.
My wife also hates motorcycles, and worries when I am out riding, but my 4 daughters have convinced her that I need to be free to do some things that I enjoy, even if there is some risk involved.
In some ways I think that wreck was a blessing to me. I found this vn750 board and other safety forums while recovering,
Check these out.
Motorcycle Safety Site
as well as some excellent books on Proficient Motorcycling (The Ultimate Guide to Riding Well, and Mastering the Ride). I believe the info on these links, and reading and re-reading these books have made me a safer rider today.
Wearing full protective gear in warm/hot weather can take most of the joy out of riding. This Bohn Under Armor may put most of the joy back. The hard armor shin guard may have saved me much pain and injury back in 2008, if I had known about it and been using it.
Good luck, and may you return home safely after every ride.