Not sure how credible this source is, but just did a google search for 'peak running age'
http://www.pponline.co.uk/encyc/0270.htm
Couple quotes for the lazy:
"An 800- and 1500m male runner is most likely to run his fastest at around the age of 25. If you don't believe it, compare the ages of past and present world-record holders for these distances. For females, the peak is delayed to the age of 27. Why women differ in this regard is a mystery. There are, of course, exceptions to the rule but these are rare. In addition, having achieved an all-time best time at these ages, the athletes often continue for another decade with world-class times. Seb Coe, for example, ran 1:41.73/800 at age 24 and 1:43.52/800 at age 33. Steve Cram ran a mile in 3:46:32 aged 25, and 3:53.8 when he was 30."
"The age at which a male runner is most likely to run a lifetime best for 3K and SK is 27, for females, 29. . For the 10K, it is 29 and 31, respectively."
"The age of peak performance in the marathon is a little more elastic-it is between 30 and 37 years of age."