Reading age
Reading age expresses how difficult a piece of text is as the age at which a typical reader could comfortably follow it. A reading age of 13 means an average 13-year-old could read it comfortably - and so can everyone older, which is the point.
It is a measure of the text, not the audience: most websites should aim for around 13 regardless of how educated their readers are, because everyone reads faster and retains more when text is simple. Silktide can express the same underlying measurement on two other scales - for language-learning contexts, and where a schooling-based scale reads more naturally. See the Readability check for how Silktide measures it.