I'm 26 but I thought I could add a couple of data points.

My father ran for many years and only stopped when he got arthritis in his knees at ~ 60. He was never a cycle commuter but he was certainly in good enough shape to be one.

My Aunt is 66 and still takes solo backpacking trips with 11 miles segments in them. She isn't a cycle commuter either, but not for lack of physical fitness (she is retired).