Personally, I've noticed degradation in various aspects of my cycling as I've aged. Balance (it's harder to keep going straight when I turn or tilt my head), vision, quickness of reaction, endurance, "snap"/acceleration, etc. All of this is somewhat compensated for by experience and increased caution/less risk taking.

At some point, the risks and/or cost must outweigh the benefits. I would suppose that the point at which it's no longer "worth it" to ride would vary widely from person to person. I know only one thing for sure: I'm closer to the end of my cycling "career" than the beginning.