The paper analyzes aspects related to the effects of the mitigating circumstance - the commission of the offence by a minor. In order for the minor of the person to be a criterion for individualizing the punishment in terms of the effects it has in this regard, as a mitigating circumstance, the notion of minor in the legal-criminal sense was defined. It was appreciated that the minor of the person who committed the offence, apart from the fact that it is a mitigating circumstance, must produce special essential effects, when individualizing the liability and the punishment, related to the particularities of the criminal responsibility of the minor, or, it is necessary to take into account the fact that age is an individual feature of the juvenile offender that indicates the level of his bio-psycho-social development. It was established that it is necessary to establish in the criminal law a separate institution with relative autonomy, comprising a group of criminal rules, which determine the special essential effects of individualization of liability and criminal punishment of minors, therefore, to establish criminal rules that to regulate clearly and unequivocally, the particularities of the liability and criminal punishment of the minor, different from those established for an adult, although there may also be common particularities.