I hit my PR's at age 21 and was running for 9 years up to that point. I believe I have more potential if I was training at the same level I used to.
But dont be weary, I have seen some people PR into their 30's.
Look at Meb Keflezghi, he is 39 and the best marathoner the US has.
I believe Bernard Lagat set the American 5k record at an age over 30.
I have also seen new runners who were never runners in college or high school pick up running in the mid 20's and after a couple years training drop a sub 2:30 marathon.
So, 25 typically has been known to be the peak age for performance, but there are plenty of examples of exceptions!
Hope this helps!
shoot me an e-mail if you have any questions!