How can one maintain cycling fitness as one ages? As I approach my mid-50's, I'm looking forward to a few more decades of cycling if all goes well. How can one avoid and/or mitigate age related issues in order to maintain cycling fitness?

I've thought a lot about this and already engage in several practices. I'm sure that I can always learn more.
Just to get started with a couple of things I already do.


Strength training with some emphasis on lower body. Since cycling is not a weight bearing exercise, older cyclists are at a greater risk for pelvic, hip and leg fractures. 
An advanced dynamic flexibility routine in order to maintain mobility, coordination and balance.


Update
As noted by darkcanuck's comment below, cycling as a cause of bone loss may very well be incorrect. In the article, Dr. Mirkin advocates vitamin D supplementation to prevent bone loss, and I've also read other sources recommending the same thing. So, probably a good idea especially for those in Northern climes. 

As for strength training, I have other reasons for how it's been beneficial for cycling as I get older.


One is that a few years ago I was experiencing considerable neck/shoulder pain on long rides. At the time I was spending long hours at the computer and cycling was my dominant physical activity. I came to the conclusion that my neck/shoulder issues were posture related and so developed a strength and flexibility program to correct my posture. In doing that I have been able to eliminate the pain and now ride comfortably on long rides.
Another issue that developed a year or so ago was knee pain. After seeing my doc and a physical therapist, I was prescribed a new 'hip' focused addition to my strength routine. Apparently I had some muscular imbalances related to cycling. Since starting this new routine 6 months ago, the knee issues are significantly improved. 


For further info on strength training for older, and even younger cyclists, here's an article from Velonews.