When searching marathon news around, I found a post talking about USA marathon performance-2013, the results was quite interesting, such as:
1. 47% marathon finishers aged 40 or above;
2. Median age overall is 36 and mean age is 39;
3. Median finishing times increased from 1980 till 2013...
More you would find in ucanrunfaster.com/usa-marathon-performance-2013