The plural of anecdote is not data, but, lacking any credible data for things like this, I'll offer myself as an example of why there is no fracking way 50% of the population could break 5, even if they devoted their lives to it.
On the first day of junior high, 7th grade for me, half of all the 7th grade boys - maybe 200 - raced 600 yards. I blew the group away. I had had a very active summer, playing a lot of soccer etc. Inspired by that, I went out for track the next spring and ran every year after that. Now I wasn't very diligent in those first years in the off-season, but I ran, at least semi-consistently, and breaking 5 minutes was a pretty big goal for me. I didn't accomplish that goal until I was a sophomore. I had trained pretty hard for it, actually in 9th grade, but was not able to do it. Now some of that may have been physical maturity, but not all of it.
Like others who have commented here, my high school was huge and our XC team was a power house. Yet, out of 4000 students every year there were no more than 8 or 9 who broke 5. And lots of kids tried.
So, I'd say that the actual percentage of males with even the potential to break 5 is likely less than 5%. Probably a lot less. As someone said above, the percentage of females who can do it is effectively 0. So, my guess is that the percentage of those with the potential to do it is around 2.
The percentage of those who could actually do it now is probably 1/100 th of that.