The proportion of drugs for use in juvenile patients is much less than for the adult population. This is due both to the lack of specific drugs for a number of childhood diseases, and the need to conduct special clinical studies in different age groups to assess safety and efficacy parameters. When developing a program for clinical trials of an orphan drug, ethical aspects of the participation of underage patients are considered, taking into account the current international and Russian legislation. Obtaining the informed consent of a minor patient from one of the parents requires detailed prior information and the establishment of a trusting relationship before the participation of a minor patient in a clinical trial. The results of clinical and observational studies of orphan drugs on the example of moroctocog alfa in previously treated pediatric patients with hemophilia A in different age groups contribute to an increase in the arsenal of drugs for the treatment of orphan diseases in the pediatric population and determine the optimal conditions for the use of moroctocog alfa in different age groups.