I ride with two different groups and the B rides have a good number of people in their 50s and 60s and a couple even older. While overall health can vary widely by our age, there is no standard that you must slow down at a given age. I'm turning 50 and I'm in substantially better cardiovascular condition than I was 20 years ago and I'm still seeing slow gains in average pace and endurance. I've had some health problems but so far they have just been motivators to get in even better shape. I'm just sad that I didn't figure this out in my teens or 20s (when I ate whatever I wanted and exercised when I felt like it) who knows how much better off I would have been.