Good question, especially for me [see my most recent post]. While I've been running competitively since 1973 I've only run 25 of those years. Also, unlike many competitive runners I didn't put in a boat load of miles in my prime. In fact, I never ran over 1000 mpy when I set all of my PB times. Obviously, those are all soft. And only ran 2000 miles for the 1st time in 2005. Since then I've run over that total every year and during 2009-2011 averaged 3500 mpy.
Most of the runners I know that ran a lot of miles in their prime are no longer putting in high mileage. Most master runners and all of my training partners from 15 years ago are running considerabley less, and for the most part are all are running much slower, whereas my half maathon time is only 4 minutes slower.
I'm 55 YO and recently retired, so I have the opportunity to run more. Not sure if I'll run more than I have the last three years [I'm only running 50 mpw now] but the body seems to be holding up pretty well if I choose to go that route.