Glad you asked. We analyzed the finish times by age group, comparing results of 43,000 New York City Marathon finishers and 15,000 New York City Half Marathon finishers, and the answer is, on average:
[Want a second opinion? See our New York City Marathon Average Finish Times by Age Group; for further comparison, see our analysis re: predicting Marine Corps Marathon Finish Times]
Here's more detail about finishers and finish times by event and age group that went into our analysis. For more, see our complete New York City Marathon Coverage, and for even more, see our comprehensive archive of 200+ Stats/Results/Analysis articles.