I was looking through my old High Level Campaigns book (from 2e) and my favorite part is a small section dealing with demographics. It works with the following assumptions: an adventure needs at least a 15 in one stat, and no stat lower than an 8 [kinda like the elite array]. Assuming everyone rolls 3d6 for stats, 1 in 10 people meet these requirements (it suggests that different stat-rolling systems simply help to guarentee that PC fall into this 10%, as everyone else uses 3d6 or an average of 10s and 11s for stats). It then assumes for the sake of argument that only half of adventures make it to the next level (so that there is a geometric fall-off). It then produces a table of the population requirements for each level of character being equal to 10*(2^level-1). So 1 in 10 are 1st level, 1 in 20 are 2nd, 1 in 40 3rd, and so on. This means that a population of 5,120 would expect a 10th level character, and a population of 1,310,720 would expect an 18th level character. So 18th level is literally 'one in a million.'
I checked this numbers against the highest NPC level stuff for building cities in the DMG (looking at only PC classes, not NPC classes). It matches up in a way, with at least the required size of a city matching the needed population for that character level, even if the averages didn't quite work out.
I always thought this was interesting, as it helps to answer how common your PCs are. Like my group of 5 level 6 PCs would be 1 in 1600. Also I was trying to use it to answer the question: how many wizards are there in a given population with the ability to make all those +1 swords that some people seem to think need to be littering the countryside. Are magic items in the "normal" game too common based on the number of people who would/could/do make them?