Interesting question, but not specific enough. Really there are six variables to your question, even more.
% of men today, no training, just every american hits the track. I'd say very, very, very small. I have a pr closing in to 4 flat and I still run 5 days a week and I know I couldn't break 5 today.
% of men and women today, no training, just everyone hits the track. Even smaller because very few women (50% of your population) can break 5 even when properly trained.
% of men at their prime. I would say 1.0%. This is based on my hs pupulation seeing that almost all americans were at their physical prime in hs. We had 550 boys at my HS and only 5 TOTAL could break 5 on any given day. I'd say most people who can't break 5 in hs never will after and an insignificant number of middle schoolers can break 5.
% of men and women at their prime. 0.55%. We had 1,100 students at my hs, 5 boys could break 5, one girl.
% of men if properly trained from birth. Who the hell knows. This would be a total guess. It could be anywhere from 100% all the way down to 1%. If I had to guess I'd say around 30%.
% of men and women if trained properly from birth. Once again, no one knows. If I had to guess, seeing half the population are women, I'd say 10% of women could, 30% of men, so 20% total population if trained for the mile from birth.