"Sometimes you may want to limit your search results to a specific reading level. For instance, a junior high school teacher looking for content for her students or a second-language learner might want web pages written at a basic reading level. A scientist searching for the latest findings from the experts may want to limit results to those at advanced reading levels," suggests Google.
Right now, this feature is only available for English web pages, but Google doesn't mention if it uses the Flesch–Kincaid readability tests.
According to Google, less than half of the indexed pages are written at a basic reading level, half of the pages are written at an intermediate reading level and about 2% of the pages are written at an advanced reading level.
{ via Search Engine Roundtable }