I reluctantly reached the same decision recently after almost 55 years of riding. The realization that it was time came slowly. The first indication was how often I was riding. In years past I would ride in temps from mid 20's to 110 or so. I found myself avoiding the extremes until I was only riding when it was comfortable, something that never entered my mind before. I also found myself thinking about how lucky I'd been. Except for a couple of broken ribs a few years ago, suffered when trying to take a friends KLR down a rocky downhill that over matched both the KLR and my non-existent dirt skills, I had been accident free.
Looking back though I think the biggest thing was my wife. As I almost always rode alone, and favored the more desolate areas, I knew in the back of my mind that she worried. But, bless her heart, she never said a word. Only after I decided to sell the R3T, and not to buy a replacement, did she acknowledge her fears.
I do miss riding and often think that while I'm still physically able, I'd sure like to get back on a bike. But, as I suspect we all experience at some time or another, that little voice in the back of your head telling you, remember all the good times and consider yourself lucky to have come out unscathed.
The decision is not an easy one, and you'll likely second guess yourself. Just know, you're not alone, its a decision that all of us have to face. Its better to make it a day too soon than a day too late.