How much does age really matter for collegiate distance runners?
Bekele won double gold at World XC at age 19
Kipchoge won 5k Worlds gold at age 18
Wanjiru broke the half marathon world record at age 18
Gebrselassie won 10k Worlds gold (and 5k silver) at age 20
Komen set his legendary 3k World Record at age 20
El Guerrouj won 1500m Worlds gold at age 21 (after tripping the previous year)
Cheptegei won 10k Worlds silver at age 20
Ingebrigtsen...don't even know where to start with him
And this list leaves off some of the more unlikely age performances (e.g. Kiplimo 13:24 at age 15) in favor of some of the greatest older runners (Kipchoge, Gebrselassie) and others who, by circumstances, are relatively more likely to have correct ages (Ingebrigtsen, Bekele).
In addition, there are cases (although rare) where 18 or 19-year-olds have exceptional performances (Kennedy, Young, many others). For the most part, they remain at or near the top, but don't dramatically improve from year-to-year.
I would argue that under-20s are at a disadvantage in the NCAA, but the difference between a 20 or 21-year-old and a 24 or 25-year-old is nonexistent (and some runners peak at 21). So an athlete competing in the NCAA from ages 21-25 has an advantage over an athlete competing in the NCAA from ages 18-22 only in that the former has four years at peak level while the latter has only two. But there's not a lot of evidence that a 21-year-old Senior is at any sort of age-related disadvantage to anyone else.
(It may be different for women; I think you'd have an argument there. But it seems to me that most of these arguments are about the men)