well, I'm still young (23), but my times have progressed quite dramatically since high school.
My junior year of HS I ran 20:04
Senior year, I ran 19:20
Freshman & soph year of college, 19:30 (junior college)
Junior year, after 1 month at a real college I ran 18:25
Senior year, 17:24
first year out of college, I ran 16:59
As cheesy as it sounds, I think it turned out perfectly. In high school, I wasn't all that fast, but my coach instilled in me a love for running and if not for him I wouldn't still be running, and I wouldn't be running the times I am now. Yeah, it took a long time, but I think that's the way to go. I gradually built up my mileage after JuCo, worked hard for a few years in a row and saw some huge improvements.