I did one of those "official training runs" for a city race a couple of months back and my pace group picked up a post-collegiate guy in his early 20s and a high school cross country runner.  At some point we broached age grading in running and I mentioned that I was 35 and the high schooler goes, "no way" and I said yeah man, born in '83.   He thought I was the same age as the 24 year old guy.  Turns out I'm the same age as this kid's father. Amazing what staying fit does to aging.