The age of eighteen has been recognized as the age of legal adulthood in most of the world. Anybody under that age is considered a juvenile. He or she cannot be given responsibilities that are given to adults, and they cannot be held liable for things the same way adults are.
Brain development psychology states that there are significant differences in the brains of adolescents in comparison to adults. The adolescent brain has asymmetric or asynchronous development. The socio-emotional system, which commands emotions and reward sensitization, develops faster than the prefrontal cortex. So, there might be great emotional impact and behavioral changes to the surrounding environment and background.
The prefrontal cortex takes time to mature, which can go on till early adulthood. This region is responsible for all the planning that takes place. It helps in controlling the impulses and thinking everything through before acting. Further, due to the change in the limbic system, the need for sensation seeking increases greatly.
The brains of juveniles cannot operate like adults in key areas. Still, in legal systems, many a times a juvenile is held guilty on the same level as an adult would. This contradicts the whole legal separation of juvenility and adulthood, and also doubts the historically established brain development and psychological theories.
Reference List
Bigner, J., Grayson, T. and Tokura-Gallo, H. (2017). Introducing Developmental Psychology. s.l.: BVT Publishing.