Since I started retiring at age 30 (work a year - take a year off), most of my good friends were very active retired people from age 55 to 75 (because I could not find any 30 year olds to hang out with). The honest truth as I gathered it by speaking to these people is that most felt a major decline in physical capability around age 70. This is the average. One of those people still races cyclocross and his DAD still races road and cyclocross in his 90s. Is he blazing fast? No, but he is still doing it and enjoying it. I have another friend in her mid 90s who is not a cyclist, but could certainly do it. She just fast-walks for exercise and travels all over the world regularly.

So my take, given my experience, is that if you take care of yourself and NEVER stop doing athletic things, you will likely have "game" into your early 70s. After that, expect to slow down and smell the roses a little more. You should be able to enjoy cycling until very near the day you drop dead, assuming you don't get some other injury or condition that trips you up.