Every four years the next President of the United States delivers an Inaugural Address to the American people. The charts below (larger version of bubble chart here) shows the reading level and the length in words of every Inaugural Address from George Washington in 1789 to Donald Trump in 2017. The reading level is measured by using the Flesch–Kincaid readability test which is calculated on the basis of word length and sentence length. The complete dataset can be found here.
The reading level has varied considerably between presidents, but the overall trend is that of decline ever since the Founding Fathers. The Address with the greatest reading level was delivered by John Adams in 1779 (28.5 grade level), while the most easily understood Address was delivered by George Bush in 1989 (6.2 grade level). The length of the Inaugural Address has varied considerably as well, but with no overall trend. The longest was delivered by William Henry Harrison in 1841 (8,460 words), while the shortest was delivered by George Washington in 1793 (135 words).
A similar declining trend in reading level can be observed regarding the State of the Union Address as well as the opening of parliament speeches and New Year’s speeches of Scandinavian Prime Ministers (in Danish).