It is not a freak of nature. Carlos Lopez set the world record at the age of 37. I've met numerous runners that did not PR until turning masters age. There are two main reasons the myth of age still persists:
* people get tired of running competitively and do not train as seriously anymore as they get older
* poor health habits result in premature aging
Now here is a a curve ball to trip the myth.
http://en.wikipedia.org/wiki/Merlene_OtteyAt the age of 44 she made the Olympic semil-final in 100 meter dash, and was only 0.03 off from making the final. Her time was 11.21. Her life time PR is 10.88. At the age of 48 she ran 11.83. Now we are talking TOP END SPEED digression of somebody who used to be FAST , not the marathon speed!
A good site to get an idea of what a senior athlete can do:
http://www.mastersathletics.net/