These are the main factors. Training for middle distance over 50 requires a persistent but cautious approach. Pete Magill has written about a rationale approach for masters runners. Pete is also an example of a runner who is not ranked as a 50 year old miler but who could easily run a world class time. There are more runners who could do it than are ranked but probably less than 100.
The training approach is somewhat different than what worked in HS or college. You need to be creative. I use a combination of cross training, treadmill and trail running to stay healthy and fit. The most difficult part is doing quality track workouts alone which I never had to do when I was younger.
I agree that if one can control lifestyle changes that occur in middle age, it is possible for most to run times similar to junior year in HS (sub 4:40 for me now and then). Although it is much more challenging at this age to diet, find time to train and sleep enough while taking care of children, parents and a buisness.