Robert J. Stevens, et.al. have done a pretty serious study of this question, based on computer analysis of texts, and here is their key conclusion:
From about 1930 through 1960 or 1970, the cognitive demands of reading curricula changed little…
In the period of 1970 through 2000 we observed a fairly consistent increase in the difficulty of reading text and comprehension tasks, particularly at third grade.
For sixth graders, however, reading texts were somewhat more complex in the 1910-1930 period.
I also found this comparison interesting:
In the 1920s, 45% of all questions asked were explicit detail questions, whereas by 1990 and 2000 curricula, that had diminished to only 8% of the questions asked.
Alas I can no longer remember who deserves thanks for this pointer. Here is an earlier Alex post about whether TV shows are becoming more complicated.