JEH,
Usually your opinions with me are like BUTTA, but I'm going to have to seriously disagree with you here.
I know that what people DO and what people CAN do if trained properly are two different things, but even so, gotta tell you, there's NO WAY 50% of American men (high school and older) can run a sub 5 minute mile. I think that for some of us who have averaged that pace for several miles that it seems like it's no big deal, but it really is.
Some things:
1) Only 5% of Americans adults can run a mile without stopping AT ALL. Of course if you trained more people, then maybe 95% could, but still, that's a big thing to consider. I, of course, have run WAY under a 5 minute mile (4:32 in high school and then capable of faster than that in college though I never raced a mile or 1500 again in peak shape), but I've had periods of time when I let myself go, gained 50 pounds, and I could still step out the door and run 10 miles or more if I wanted to (not without being really sore the next day though). I could do this because I have some natural talent. MOST people do NOT have the talent to run even a 7:00 mile let alone a 5 minute one, and there's a WORLD of difference between those two times.
2) When I was in high school, there were 40+ guys on the cross country team. Most of those guys were there for 4 years -- that's what they did. They ran. They trained. My senior year, there were 3 of us on the team out of 40+ who could break a 5 minute mile -- that's 7.5% maximum of high school aged guys who ran a lot and were on the cross country team in part because they were good at running. I ran with a couple guys in college who could not run a sub 5 minute mile.
3) I've talked to many a runner -- runners who have been running since they were 20 -- running for 30 years. They are fit and lean and run 70 miles a week including more intense days and so on, and they can't run a sub 5 minute mile.
I think oftentimes people here don't count their lucky stars enough that they were given some talent to run fast. Lots of fit healthy thin people couldn't run a sub 8 minute mile to save their lives, let alone a sub 5 minute mile.
While I think we established that between 1-2% actually DO run faster than a sub 5 minute mile (though I think it's even less than that), I bet the max would be that 5% could even after a few years of intense proper training. It's just not that easy.
I'm sure your next post will be once again like BUTTA! :)