The "reading level" or "grade level" of text is an interesting idea.
Take a look at the table "Grade Level and Circulation of Current
The "grade level" numbers refer to US school years, so 12 = last year of
high school, 16 = last year of college.
Microsoft Word will compute readability scores for you. This article
about website accessibility describes some of the pluses and minues of
the MS Word readability function.