One of 'em wrote:
What do the masses say? And please keep in mind the difference between "respectable" and "good."
I'd say anything under 20 fits the bill for "respectable." At 80 percent of road races, that'll earn you an age-group award.
Good? Gotta go under 18:30, at least.
I am 31, and I began running races when I was 26. I ran sprints in HS, so I am not totally foreign to the sport. If you're looking at regional races across the country, anything sub 17 is going to be up there with the fastest times, unless it's heavily loaded with former D1 runners, still in their early to mid 20s. Past the mid to late 20s, the times begin to drop, as life gets in the way and the volume of training can take a back seat. I can always tell if somebody in my 30-34 class was a college runner, typically ripping off a high 15-low 17 range and just looked super efficient. That's a product of several years of training. The time difference between late 20s to late 30s is pretty significant for the most part. As the speed begins to drop, the endurance and efficiency picks up, as why it's pretty common to see Masters Runners continuing to run strong times well into their 40s, and 50s. A fellow coach of mine at 53 was still running mid to high 18 5ks, say what you want, but that's freaking strong and I can only hope I can hang on another 20 years like that.