In the past several years, there has been broad media coverage of a push to redefine the end of adolescence as 24 years old rather than 19 based on studies of brain development in young people continuing into one’s mid-twenties. What does that mean scientifically, and does that correlate to to the onset of adult cognitive patterns?
When scientists refer to brain development being complete in a persons mid-twenties, they are referring to the time when the prefrontal cortex is fully developed. The prefrontal cortex is believed to mediate the highest-level cognitive processes such as executive function, language, and intelligence. The brain is thought to mature in a back-to-from pattern, leaving the prefrontal cortex as the last region to fully mature in humans. This delayed development of the prefrontal cortex relative to other regions of the brain has been observed in longitudinal MRI studies of adolescence.
The structural maturation that occurs in the brain during adolescence consists of two major components: dendritic pruning and myelination, represented on an MRI as decreased gray matter and increased white matter, respectively. Dendritic pruning refers to the elimination of unused synapses following the proliferation of these connections that occurs just prior to puberty. Myelination is the process by which neuron fibers become more insulated with myelin, thereby becoming more efficient at transmitting signal.
In studies of adolescents, these structural markers of brain development are also correlated with more mature behavior. Does this mean that a person is totally behaviorally mature once the prefrontal cortex is developed? Some experts are uncertain. In a 2009 article for the Journal of Adolescent Health, scholars from Johns Hopkins and the National Institute of Mental Health argue that the study of human brain development in adolescents and young adults necessarily relies on non-invasive neuroimaging techniques such as MRI that only provide a limited view of neurodevelopment. In addition, studies that seek to link benchmarks of cortical development to behavioral maturity are limited in that the behavioral tests that can be carried out in an MRI are vastly different from the nuanced challenges that a young person will experience.
Advances in neuroimaging and behavioral assays will fine-tune the scientific understanding of human maturity. In the meantime, it is clear that while scientists cannot clearly define adulthood, it is even more complex than was previously believed.