weird age gap in race results

I think so. Folks in their 20s-30s are busy starting careers and families. By the time they're 40, mortality starts creeping its ugly head and they have more free time to train.

I would say the bell curve for age at races in my area is probably the highest at 35-45. Having kids definitely gets in the way of training and racing, especially when they are elementary to middle school age and have a lot of activities. But I also think a lot of people find distance running when they are in their late 20s to early 30s. They train seriously for a few years and start hitting good times as they get into their late 30s and early 40s. The 50+ runners are often empty nesters and are very social in the running scene. The running clubs in my area are more than 50% in the 50+ age group as far as active membership goes. This is actually a big change over what it was like 20-30 years ago.