Adolescents reach physical and sexual maturity before their brains finish growing. The gap between sexual maturity and cognitive maturity is wider for girls than it was in the late nineteenth century. What is the reason? Brain myelination takes place later than it did in earlier generations. Girls reach puberty sooner than they used to. Today's children aren't given the responsibilities they need for optimal brain growth. People's brains develop longer due to the increased complexity of the environment.