No single aspect or characteristic of text can be used to evaluate reading material. In placing a text along a gradient of difficulty, many factors are considered. Books for beginners will have just one or two lines on a page. Sentences begin on the left and print is clearly separated from pictures. In more complex books, sentences begin in the middle of lines or are carried over onto the next page.
Fonts become smaller.
Early books have simple plots and some repetition. Some books use repeating episodes or complex plots organized chronologically. As books become more challenging, more interpretation will be needed. Easier books provide pictures to support the reader in gaining meaning and solving words. Picture support gradually decreases as you move up the gradient of difficulty.