How common is college graduation at an early age? I am interested in the statistics of early college graduation, or more generally, the statistics of extreme ages in academic settings (highschool, college, grad school, etc.). For instance, how many students who earn a college degree graduate one, two, three, or more years earlier than the typical age of 22 or so? I am willing for any statistics to qualified in any way (percentages in country W, at university X, in state Y, or from year Z). A quick Google search does not easily reveal this information.

I estimate that less than 5% of the population graduates college two years earlier than normal based on my acquaintances, but this is likely biased as I am a graduate student and it may be smaller.

EDIT: It occurs to me that my question does not ask for any opinions on whether early graduation is good, neutral, or bad. Perhaps it would be interesting to expand the question and have those who experienced graduating early give their stories or opinions.
At least in the USA, the number is probably much smaller than 5 percent. 

The reason for this is that there are generally fixed lengths for education, and minimum enrollment ages at which the process can start (at least for publicly educated students, who are still the majority).

Finishing two or more years ahead of schedule means that you probably have had at least two events that belong to the following categories:


Started education a year earlier than "normal" (perhaps because of birthday-limited enrollments)
Skipped a grade during primary or secondary (high-school) education
"Accelerated" college studies by reducing the expected enrollment time by either a semester or a full year (through early accumulation of credits via work in high school, or taking college placement exams, credit overloading, and other methods)


The first is the most common, but still only applies to about one-third of the population. The others are much less frequent, with the second probably pertaining to only about 1 percent of students (if that many). The third also probably is not that common, but I don't have hard numbers (but again, probably less than 5 percent of college students finish in three years or less!).

Now remember that you have to have at least two such events, and you can start to see why the odds are stacked against a 5 percent rate.