The Guardian has an interesting graphic showing the duration and reading level of every SOTU since the nation’s founding. I can’t paste it here because it’s interactive so you’ll have to click over and take a look.
It seems that the reading level of the SOTU has been on a long steady decline with more recent Presidents at the bottom of the pack. President Obama’s SOTUs are at a 9th grade reading level on average, though it turns out 2009 was an outlier. His three other SOTUs (the current one hasn’t been rated yet) have been at an 8th grade level.
President George W. Bush, routinely mocked by the left for his verbal gaffes, gave SOTUs at a 10th grade reading level.