Developing reading literacy in students with mild intellectual disabilities (ID) in the adapted programmes with lower educational standards is extremely challenging. While there are similarities and differences between reading literacy in students with and without ID, the gap between the two groups increases with age. Listening (language) comprehension of many students with ID often exceeds reading comprehension; usually even older students fail to achieve the same level of listening and reading comprehension. The explanation of reading difficulties of students with ID includes two basic paradigms. The first one relates to the overall slow cognitive development reflected in the literacy development as well. The latter is slow but it follows the same sequences as in students without ID. The second paradigm highlights structural differences in reading literacy of students with and without ID. It emphasizes different cognitive and linguistic factors responsible for reading difficulties in older readers with ID. Our study has confirmed the two above mentioned aspects with the comparison of the reading literacy in the younger and older students with ID. For this purpose, 61 fifth graders and 70 ninth graders included in the adapted educational programmes participated in the study. Characteristics of the reading literacy of both groups of students with ID were investigated with several multivariate methods and compared afterwards. We gained insight into the reading literacy of students with ID in the two important periods of elementary education (at the end of systematic literacy teaching and at the end of primary school). We upgraded our findings by comparing the results of fifth graders with the standards applicable to the students without ID at the end of systematic literacy teaching. The results show that reading efficiency of the students with ID is based on lower level processes (decoding) much longer, while the roles of decoding and language comprehension change with age similarly to the students without ID. Reading difficulties of students with ID are complex and relate to different factors of reading literacy, among which we highlight phonological awareness, auditory short-term and working memory, vocabulary and linguistic reasoning. It is hard for the students with ID to compensate their reading difficulties due to their problems with transferring knowledge to the new situations and a reduced ability to independently use experiences acquired in reading lessons. The explanation of the structure and development of reading literacy of students with ID represents an important contribution to knowledge in the field of special education and it provides an empirical basis for improving the teaching approaches. Based on the research findings we give different recommendations for teaching reading to children with ID. In addition, we present solutions on how to adapt the reading tests used in our study to detect the best student’s achievements.