According to Article 91 of the Criminal Procedure Code, the condition of criminal liability for adults under 18 years of age is subject to the attainment of criminal maturity. This fundamental research, using a descriptive and library method, has addressed the jurisprudential and legal foundations of criminal maturity. According to one view, criminal maturity is related to the field of perception. Consequently, assuming a lack of understanding of the nature of the crime, its ugliness, and doubt in intellectual growth and perfection; based on the ruling of reason on the "ugliness of punishing undocumented persons", and "narratives indicating lack of responsibility in the assumption of stupidity and intellectual weakness", and also considering "narratives denying responsibility from an insane person in accordance with the ruling and the subject", the criminal liability of undocumented persons is excluded. Also, assuming a lack of maturity, their punishment is rejected by secondary rules. As it does not seem correct to include criminal maturity in the field of knowledge and awareness because knowledge and ignorance are subordinate to understanding. From a legal perspective, since "will" and "perception" are necessary in the realization of the crime and its elements; The realization of the material element due to the lack of will to commit the crime and the psychological element due to the lack of desire for the result and criminal malice, is ruled out by the lack of development. If the existing necessities and the necessity of responding proportionately to the crimes of children, and its compatibility with progressive ideas such as "gradual criminal responsibility", and "the need for protective and educational measures in the crimes of children", "ensuring the goals of the legislator", and finally "the principle of individualizing punishment" have increased the necessity of penal development doubly.