The chapter is devoted to the problem of the prerequisites for the acquisition of a new skill -such as for example learning to read- in order to see whether these prerequisites are peculiar to this skill or whether they may be found in the acquisition of other skills at the same or at another period of development. Learning to read within an alphabetic system does presuppose the capacity for explicit segmentation of speech in terms of phonemes, which would not appear until around age 6 years. From this point of view, it seems possible to consider the capacity for segmenting a whole into abstract units as the major prerequisite for learning to read in the strict sense. Similarly the strategies for segmenting sentences are based on different indices: semantics, positions and formals (morphosyntactic). The passage from pragmatic to formal strategies also takes place around 6 years of age. Then, I have described other research areas where the age of 6 years is also typical of changes comparable to those I have defined. To conclude I have reinterpreted the process of reflective abstraction as a process of construction of new representations at different periods of development.