I stumbled onto this thread (and this website) when I was searching for something totally unrelated, but it caught my eye and I've been reading all the posts for the last hour or so.
Let me open with this: There's no way that most (50% or more) of guys, if tested out of the blue today, could run a 5 minute mile. Not even close.
I'm 29 now. Ten years ago I ran track and cross country in high school, and I want to say my best mile time was about 5:10. I never broke 5 min, and I wasn't a standout by any stretch of the imagination, but I definitely remember that compared to the rest of the male population, I might as well have been THE FLASH.
Fast forward to today. I'm in the Army, and a week ago I did the 2 mile run (part of the PT test) in 11:20. Again, not a standout performance when compared to competitive runners, but compared to the average dude...Flash Gordon baby. And that's compared to the rest of the Army too, and most of these soldiers are in shape people, not couch potatoes who haven't worked out in years.
So, in terms of percentages, here's my answer. Even if you narrowed the age range to the "prime" years of 16-35, the percentage of guys who could go out right now and break 5 min would be less that 1%, guaranteed.