My high school xc team would always do a 1 mile time trial at the end of the season. I don't remember the exact stats, but I am somewhat confident that all 50+ runners on the team would go under 6:00 and that the average would be around 5:15ish each year. Looking back at my school's archive, there wasn't a single person on my team who failed to break 20 in the 5k during our home meet my senior year. This is obviously a pretty selective sample, but it does include plenty of 14 year olds who had been training for less than a year as well as people just running xc to satisfy gym requirements.
Given this, I find it pretty hard to believe that the generic 20 year old with a few years of decent training couldn't run at least a 2:30 800. The issue is that the generic 20 year old wouldn't be willing to train decently for a couple years, so even if they could optimally run close to 2:00 there's no way they'd get to that level of fitness.