The purpose of this study is to clarify whether readability formulas based on linguistic factors are suitable for secondary and older primary age texts. A comparison among fomulas for primary age texts, some for both primary and secondary age, and some for secondary age revealed that exclusive ones for narrow age range were more effective. A model estimating readability scores from the average number of sentences in paragraphs or a model with two factors, the average number of sentences and paragraphs in texts was found to be good one for secondary age. While a model based on total number of unique syllables or a model from total number of unique syllables and new syllable occurrence ratio was good for primary age.