For me about 5 years ago when the knees gave out. Hadn't ridden since I was a kid.
I've never understood this: "A few years ago my brother in law got killed on his motorcycle. My wife said mine had to go."
If he died in a car, would you not be able to drive anymore? I've been riding motorcycles since I was 10 years old, and I will be 55 in June. I ride a GSX-R1000, and I have even run track days at the local race track, popping 150+ on the straights with it. My wife gets a little nervous when I'm at the track, but she knows it's in my blood and would NEVER ask me to give it up. I have lost friends over the years to motorcycle accidents, but also to car accidents, etc. One does not stop living, one goes on doing what you love.
Just thinking out loud again...