I think I turned the corner about two years ago. I was definitely stronger and faster at 40 than in my early 30s (because I trained a lot harder), and held my own through my mid-50s. I turned 60 in January, though, and I've definitely slowed down. I sort of feel like I COULD still do what I used to--the distance, anyway, if a few mph slower--but it's so much harder now that I have a hard time making myself do it. I'm not about to stop riding or doing anything else I enjoy, but sometimes I'm content to cruise along at 14mph rather than busting my butt.