The slowing down with age is a fascinating question but I'm inclined to think that our abilities fade with age much more slowly than is commonly assumed.
The record books are bound to show quite a steep falling off after 40-45 because, until recently, the numbers still training regularly past these ages was very low.
As for going past your parkrun time of 17:28 at age 38 it is basically there on a plate for you if you want it enough.
I ran regularly until 1993 and then gave up until 2008, annual parkrun PBs since then have been:
2008: 21:09
2009: 18:37
2010: 18:03
2011: 17:33
2012: 17:25 (twice)
2013: ??:??
Will I go faster at age 47 in 2013 or not? Who knows but I've got my eye on a 16:XX and will enjoy trying to get there 
If we dont try we wont know.........
Edited: 06/12/2012 at 22:18