In human cultural development, the school age is of particular importance. Its beginning is marked by the children achieving readiness to start reactive learning, i.e. learning under teacher’s instruction. At this time, the child shows the greatest responsiveness to knowledge and skills; moreover, the child has respect for adults who can help him or her complete tasks which he or she cannot do on his or her own. According to the assumptions of the cultural-historical trend, learning under instruction is the leading form of children activity in this period. It means that the child satisfies his or her most important developmental needs within this activity, and that the central developmental processes take place at this time. Thanks to this activity, in the school age the child satisfies the need for productivity. Correct diagnosis of readiness to learn under instruction is, therefore, crucial for the child’s cultural development. It makes it possible to launch activities aimed at supporting it at the right time in the process. The experimental method may be more useful in this respect than other diagnostic methods. It allows for identification of means for supporting a child in the process of completing tasks which at the moment exceed his or her current capabilities. The experimental form proposed by Lev Vygotsky, known as the double stimulation method, appears to be particularly useful for diagnosing children readiness to learn under instruction.