Everyone is different. I had a rather sudden 5% reduction in average speed at age 47 (riding the exact same routes for many years). When I moved to Colorado at age 50, it took a couple of years to become accustomed to lots of climbing. I rode my fastest time from Idaho Springs to the top of Mt. Evans at age 53 (2:35). Every year after that, I knew that my fitness was not good enough to better that time and haven't ridden it since.

Just a few weeks ago, I found out that my left knee joint will need replacement within a few years, so I'm quitting. I'm getting treatments to reduce the pain and stiffness until the replacement. Hopefully, I'll make it to 62 ( five more years) before the knee is replaced.