OP,
As a runner in the 55-59 age group, I can say that we are the tail end of the running boom. We are the last of the age group that remember Frank Shorter winning the gold in the Olympics. Rodgers on CBS Sports, live, winning Boston and NYC. Being transfixed on the television for a live coverage of the Falmouth Road Race. So, we are now part of the dying breed. The last of the group that believed in mileage, mileage, mileage. The routine in my age group was always the same. Daily miles during the week. Long runs on weekends. Limited selections of race, meant that almost every race you attended, so did the number one runners in the area. And, this made the races more competitive. Today 17:30 wins one of the many local road races. When I was in my 20's and 30's in the 80's and 90's, I would run 15:40, for example, and walk away with nothing.
So, in short, with such a large volume of runners from the running boom, the chances are much higher that there are still a good amount remaining that are still into it. Therefore, you see some good and even very good times in races from guys 55-59. Soon, we too will start to fade away, and the volume of hard core runners will shrink. On the rise will be the runners demanding two water stops in a 5k. Finishers medals for a 5 miler, taking selfies while in the race, and worshipping those that actually run any pace that starts with a 6.
As for your claim that there is a drop off at 60, I can say that I fall into that category. I ran 30 marathons, 18 of them under 2:30, and at this point, I really feel beat up, and don't really have any desire to do the required training to go out there and run a marathon. Maybe if I can just jog a marathon, and not feel the need to try to have a good finish time, then I might be able to accept running a 3:40 or so. But our group will be gone soon, and then America's distance running will give way to the college runners who have become Africans running in our country due to being recruited.
My hope is that there might be a real second running boom. one without demanding water every mile and a half. One where runners push themselves due to competition from other local runners.
Bottom line; I'm glad I grew up when I did. Outside, running, playing, coming in when the street lights are on. Went to my friends house five or six miles away, on my own.
Let's hope there is a resurgence of hardcore distance runners in America. Want to make America great again? Have an American win the Olympic marathon! Bring back great American distance runners!