The average American adult reads at the 7th-8th grade level. A lot of information written for the general public – like patient education materials – is written at this level. Unfortunately, a substantial number of Americans have trouble comprehending even these simple texts, because they are barely literate. Roughly 45 million US adults read below the 5th grade level, including 19% of high school graduates and half of adults under the poverty line .
Using a different metric, about 30 million US adults are “functionally illiterate”, with “below basic” literacy skills. This means if they're able to read at all, it's only at the most simple and concrete level. And by 'read', I mean mostly recognize words and locate information in short texts, not draw inferences from reading material. Very little connecting of dots.
Another 60 million or so of US adults read at the “basic” literacy level, which is sufficient for reading and understanding information in short, commonplace texts, like government pamphlets. A “basic” literacy level is comparable to a 6th-8th grade reading level, although it's not exactly the same because literacy level and reading grade levels are different concepts.
Literacy is defined as: “understanding, evaluating, using, and engaging with written text to participate in the society, to achieve one's goals and to develop one's knowledge and potential.” Pretty close to what we used to call “reading comprehension”, except that the concept of reading grade level is a bit vague and doesn't have clear functional implications, while literacy levels, and their functional correlates, have been the subject of much research and have been specified in great detail.
I still think there’s a place for referring to grade levels (at least in blogs). Literacy levels can be confusing to non-specialists. Saying that 45 million Americans read at "below the 5th grade level" is clearer and packs a greater punch than saying they read at a "below basic" literacy level. You get the picture: Jonathan can't read - or at least not very well.
References:
Centers for Disease Control: Understanding Literacy & Numeracy. https://www.cdc.gov/healthliteracy/learn/understandingliteracy.html Accessed 3/27/2017
Literacy Project Foundation: Staggering Illiteracy Statistics. http://literacyprojectfoundation.org/community/statistics/ Accessed 3/26/2017
Medline Plus: How to Write Easy-to-Read Health Materials. https://medlineplus.gov/etr.html Accessed 3/27/2017
National Center for Education Statistics: Demographics. https://nces.ed.gov/naal/kf_demographics.asp Accessed 3/27/2017